One Hundred Years of Caste Determination in Hymenoptera
Diana Wheeler, Department of Entomology, University of Arizona, Tucson, AZ 85721
Contacts:
Phone: 520-621-3273
Fax: 520-621-1150
Email: dewsants@ag.arizona.edu
The manuscript is based on a plenary address given at the XIV International Congress
of the International Union for the Study of Social Insects held in Sapporo, Japan, August
2002
1
Introduction
In this paper I place our understanding of caste determination in social insects
into its historical context. Over the last few centuries, castes have had an interesting
relationship with the developmental of biological thought. Charles Darwin used the
existence of sterile castes, and especially of multiple worker castes, as a test of his
theory of natural selection (Darwin, 1859). Castes have continued to be seen as an
intriguing phenomenon, but one whose study tended to take place outside current
trends in biology. Today, the existence of castes still raises important questions about
development, evolution, levels of selection, and gene by environment interactions.
The history of biological thought combines history, philosophy and science. I
believe that understanding the intellectual roots of our disciplines may protect us from
the hubris and biases that have splintered biology in the past. Among the historical
analyses I have found enlightening are the works of William Provine on the history of
population genetics (Provine, 1971; Provine, 1986), Ernst Mayr on the unification of
genetics and population biology (Mayr, 1982), and Scott Gilbert on the reunification of
evolutionary and developmental biology (Gilbert, 1994; Gilbert, 1998; Gilbert et al.,
1996). And there is nothing to take place of original sources which reveal the continual
struggles regarding major issues, posturing of large egos, and perpetual talking past
each other.
Reading such works as the exchanges between August Weissman
(Weissman, 1893) and Herbert Spencer is humbling and puts our modern efforts in
perspective, revealing that we all too often cover the same ground again and again
without seeing dominant features in the conceptual landscape.
2
The age of naturalists
The period of time before scientists became aware that the earth was very old
can be called the age of naturalists. Biologists observed nature and reported their
findings. In this era, the lives and organization of social insects were described. The
existence of castes was a source of amazement, but it did not strain the current world
view. Observations of honey bees were extensive, in part because of their agricultural
value and in part because of their intriguing societal organization. Franz Huber, a Swiss
naturalist and beekeeper, emoted about bee castes
“this partition of industry and courage, on the one hand and of prodigious
fertility, on the other – this partition, originating from the mysterious rearing of
the larvae, is among the finest subjects for contemplation which natural
history affords” (Huber, 1821).
A major challenge for naturalists was how to organize biological diversity, as
expeditions of exploration brought more and more specimens back to the museums of
Europe. The most satisfactory system of organization at the time was one developed
by Karl von Linné of Sweden, the one we are so familiar with today. This system was
based on typology, in which a representative of each organism was described as the
Type of the group.
Insects with physical castes did not represent a problem; a type
member of each caste was simply described as part of species’ descriptions.
Darwin’s greatest difficulty
Charles Darwin’s scientific training took place within this tradition of natural
history. By the early 19th century when he was developing as a biologist, advances in
geology and paleontology made it clear that the earth was very old and that the species
present on earth changed over time.
Clearly, evolution had occurred.
A major
challenge to the day’s natural historians was to posit the mechanisms that could have
shaped species evolution over such long periods.
3
Lamarck was one well-known
architect of a theory that explained such changes: the inheritance of acquired
characters. After extensive travels and decades of contemplation, Darwin proposed his
theory of natural selection for the origin of species. Darwin was the first to realize that
the existence of sterile castes would be a test for any theory of evolution.
If evolution
relies on passing on traits from one generation to the next, how can sterility evolve and
be maintained? A famous, and too often quoted, passage of The Origin of Species
relates Darwin’s concern that the existence of castes in social insects posed a special
and perhaps fatal difficulty to his theory. The special difficulty actually involved three
issues. The first was that, at least in terms of direct reproduction, the trait of sterility
cannot be transmitted to the next generation. Darwin soon realized, however, that
“this difficulty, …disappears, when it is remembered that selection may be
applied to the family, as well as to the individual, and may thus gain the
desired end” (Darwin, 1859).
So, using the concept of families or lineages that share genes, he suggested that
inability of some individuals to pass on traits directly did not mean that these traits were
lost – as long as some members of the family or lineage were reserved to reproduce, as
in domesticated animals or plants that are regularly harvested. Second he argued that
the tendency for fertile individuals to produce some sterile offspring could be
transmitted.
“Thus I believe it has been with social insects: a slight modification of
structure, or instinct, correlated with the sterile condition of certain members
of the community, has been advantageous to the community: consequently
the fertile males and females of the same community flourished, and
transmitted to their fertile offspring a tendency to produce sterile members
having the same modification…and that by the long-continued selection of
the fertile parents which produced most neuters with the profitable
modification, all the neuters ultimately came to have the desired character. .”
(Darwin, 1859)
4
Third, Darwin noted that his theory could even account for the existence of 2 discrete
sterile castes, the greatest of the difficulties.
“I believe that natural selection, by acting on the fertile parents, could form a
species which should regularly produce neuters, either all of large size with
one form of jaw, or all of small size with jaws having a widely different
structure; or lastly, and this is our climax of difficulty, one set of workers of
one size and structure, and simultaneously another set of workers of a
different size and structure; a graduated series having been first formed, as
in the case of the driver ant, and then the extreme forms, from being the
most useful to the community, having been produced in greater and greater
numbers through the natural selection of the parents which generated them;
until none with an intermediate structure were produced. Thus, as I believe,
the wonderful fact of two distinctly defined castes of sterile workers existing
in the same nest, both widely different from each other and from their
parents, has originated.” (Darwin, 1859)
Sterile castes were certainly not fatal to Darwin’s theory, but they were to Lamarck’s.
One part of Darwin’s argument was that the sterile workers were modified from
reproductive forms. H. Dewitz in Germany reasoned that if this was true, one might be
able to detect relict structures of queens in developing workers.
Using histological
methods, he was able to show the presence of wing discs in workers of several species
of ants (Dewitz, 1878).
The existence of relict features in workers has continued to provide interesting
insights through the present day. One hundred years after Dewitz’s work, we showed
that soldier larvae in Pheidole vinelandica had rather large mesothoracic disks that
begin growing about the time soldier determination occurs (Wheeler and Nijhout,
1981a). Recently, a new investigation of wing disk development in several ant species
showed that the mechanism behind the failure of worker disks to develop was not the
same across species. Furthermore, the failure of fore and hind wing disks in Pheidole
to initiate and/or complete development involved different modifications of the genetic
network for developing wings (Abhouheif and Wray, 2002). In a second recent study,
5
apoptosis was shown to be involved in the failure of soldier fore wing disks after
eversion (Sameshima et al., in review).
Brief expressions of wing imaginal disks during worker development are an
example of caste features that seem a bit scrambled in that features of 2 castes appear
in the same individual. Another example of scrambled features occurring in normal
caste morphology is the soldier-type head in queens of Camponotus (Colobopsis) and
many cephalotine ants (Baroni-Urbani and Passera, 1996; Baroni-Urbani, 1998;).
Mixed caste features can also be induced pathologically by parasitism (Wheeler, 1928;
Passera, 1976;) and experimentally by hormone treatment (Murphy, 1974; Ono, 1981).
A relatively recent and critical insight into the process of development is its fundamental
modularity in both signal transduction and networks of gene expression (Raff,
1996;West-Eberhard, 2002; Wilkinson, 2002). Future research on caste determination
should establish if developmental modules are trapped in single developmental
pathways, or if they can be recruited from one pathway to another.
After Darwin
After Darwin’s publication of Origins, biologists struggled to test the ‘all
sufficiency of natural selection’ in accounting for evolutionary processes (Weissman,
1893). They were hampered by a complete lack of understanding of how characters are
transmitted from one generation to the next. The field that seemed to hold the most
promise in filling this void was embryology. At the time, embryology included both the
transmission of genetic information from one generation to the next and the unfolding of
that information during development (Gilbert, 1998).
By the 1900s, it became clear that embryology was making progress in
understanding development of individuals but it was failing to address the issue of
6
transmission of traits. This failure, along with the rediscovery of Mendel’s work, split
biologists into different camps. The new geneticists studied the transmission of traits,
embryologists studied the expression of traits in ontogeny, and naturalists studied
natural populations. The field of biology was diversifying (Figure 1).
Unfortunately, many geneticists viewed their subfield as the only true one (Figure
2), often with overt religious imagery (Gilbert, 1998). This is not a healthy way for
science to grow, and major aspects this hierarchical perspective still persist today.
Geneticists of the day, led by T.H.
Morgan, believed that genetics brought the
study of evolution into the realm of science
and what remained behind represented
only failed attempts.
Early geneticists
were tightly focused on genes.
minimized
the
importance
of
They
natural
selection as an evolutionary force and
failed to deal with the biology of natural
populations. Naturally, biologists in other
fields noted that there were important
omissions and felt alienated (Gilbert, 1998;
Gilbert et al., 1996).
For caste determination, during the rise of early genetics, a key question became
whether the mechanisms underlying the development of different castes was
blastogenic (genetic) or trophogenic (due to differential feeding). To be able to answer
7
this question, social insect biologists returned to the natural history that had been
collected in the preceding centuries.
What natural history said about caste determination
Before the publication of Darwin’s Origin of Species by Means of Natural
Selection in 1859, honey bees had been observed for centuries as part of their
domesticated role in honey production.
The fundamentals of bee biology began to be
widely known in the 1500s, as beekeepers began publishing their observations. As
early as 1568, Nickel Jakob in Germany knew that worker bees can raise a queen from
eggs or young larvae. By 1586 in Spain, Luis Méndez de Torres understood that the
queen was the mother of all the bees in the colony. In the following century, Charles
Butler and Richard Remnant, both in England, established that drones were males,
queens were females, and worker bees were neuter. Contributions of beekeepers and
scientists from across Europe continued to mount in the 1700s (Crane, 1992).
Franz Huber, a beekeeper in Switzerland during the late 1700s and early 1800s
was an especially talented observer. In 1814, addressing Joseph Banks of the Royal
Society of London, he reported his findings on the production of new queens. A.G.
Schirach, a German beekeeper, developed a method for rearing new queens by
transferring young larvae from worker cells to queen cells in queenless colony
fragments. Banks found Schirach’s results on the “conversion of common worms into
royal ones“ intriguing and urged naturalists to repeat the work. Huber reported
“I hasten to inform you that all my researches establish its truth. During ten
years that I have studied bees, I have repeated Schirach’s experiment so
often, and with such uniform success, that I can have no longer the least
doubt on the subject. Therefore I consider it an established fact, that when
bees lose their queen, while several workers’ worms are preserved in the
hive, they enlarge some of their cells, and supply them not only with a
different kind of food, but with a greater quantity of it, and that the worms
8
reared in this manner, instead of changing into common bees, become real
queens” (Huber, 1821).
So, for honey bees, the formation of castes was clearly trophogenic.
Wasps also
appeared to have nutritional determination of caste, although the differences between
castes were not as clear-cut as those of the honey bee (Marchal, 1897)
What about ants? Ants have sufficiently different methods of brood rearing to
suggest that at least some ants might have genetic determination. As Forel (1894)
pointed out, ants appear to have neither different types of cells nor special brood food
that would enable them to identify future queens and provide them special food.
Therefore, it followed that caste determination in at least some ants would be
blastogenic.
William Morton Wheeler, the leading ant biologist in the United States in the first
part of the 20th century, was an embryologist by training and a naturalist by inclination
(Evans and Evans, 1970). By his own admission, he “remained on the fence in this
controversy, with an increasing inclination to drop off on the blastogenic side” until 1937,
when he committed himself to that position (Wheeler, 1937). One reason Wheeler was
slow to commit to the blastogenic view for ants was that he was contemptuous of
geneticists, whose arguments he found unsatisfying and inadequate (Evans and Evans,
1970). The evidence that finally convinced Wheeler of the merit of blastogenesis was a
colony of Acromyrmex octospinosus that had an unusually large number of abnormal
individuals that expressed characteristics of more than one caste or sex. He argued
that caste determination could not be trophogenic since characteristics of more than
one caste appeared in individuals which could represent only one nutritional history.
Despite this commitment in print, he included the caveat that
9
“the long controversy between those who maintain that .. .caste is due
merely to differential larval feeding (trophogenic) and those to maintain that
this dimorphism is already predetermined in the egg (blastogenic) cannot be
completely composed till biologists have found an escape from the older,
more comprehensive environment-versus-heredity impasse” (Wheeler,
1937).
Experiments and long-term studies with ants
Biologists eventually resorted to experiments to resolve the blastogenic vs.
trophogenic question. In the 1930’s, Pheidole pallidula was used by Goetsch to show
that new soldiers were produced only when ‘flesh’ was provided in the colony’s diet
(Goetsch, 1937). The importance of dietary protein was confirmed by Gregg a few
years later. Gregg also demonstrated the inhibitory effect of adult soldiers on soldier
determination, even when nutrition was favorable (Gregg, 1942).
In the 1950s and 1960s, researchers from several laboratories in Europe carried
out detailed long term studies that addressed queen determination in several species.
M.V. Brian and his associates worked with two species of Myrmica in England (e.g.
(Brian, 1956; Brian, 1974; Brian and Hibble, 1964)).
K. Gösswald and K. Bier in
Germany studied two species of Formica, eg. (Gösswald and Bier, 1957; Bier, 1954).
In France, L. Passera reported on queen determination in Plagiolepis pygmaea
(Passera, 1969; Passera, 1980). These studies confirmed the importance of nutrition
and environment in caste determination, and they have been summarized in more detail
in Wilson (1971) and Wheeler (1986).
So, is it blastogenic or trophogenic, or both?
Overall, mounting evidence showed the dominance of environmental factors,
including nutrition and the social cues, underlying caste determination in Hymenoptera
(Wheeler, 1986; Wheeler, 1991).
Did that mean the question “Is determination
blastogenic or trophogenic?” was settled in favor of nutrition and other environmental
10
factors? Not exactly. As W.M. Wheeler said, the question was, at its root, an issue of
nature vs. nurture (Wheeler, 1937). The falseness of the dichotomy between nature
and nurture, genes and environment, has been appreciated at least since the concepts
of phenotype and genotype were first articulated (Johannsen, 1911).
The following 4 caste phenomena may require more than developmental
responses to different environmental factors to explain them. The most often cited
example of genetic determination is that of queens in the meliponine stingless bees. In
Melipona, brood is reared in combs, as in Apis, but there is no distinction in size or
position between cells for rearing workers and queens. Given favorable nutrition, the
maximum percentage of queens produced is approximately 25% or 12.5%, proportions
that suggest control by 2-3 genetic loci (Kerr, 1950). Juvenile hormone can induce
queen development in all larvae, which suggests that the genetic differences between
castes may lie in the biochemical linkage between nutrition and induction of the queen
developmental pathway (Velthius and Velthius-Klupell, 1975).
Determination is not
purely genetic, however, since even genotypic queens develop as workers if nutrition is
poor.
Buschinger and his colleagues have provided a second example of genetic
effects on queen development in their studies of the slave-making species
Harpogoxenus sublaevis and the non-parasitic Leptothorax sp. A . In H. sublaevis, they
discovered the presence of a genetic factor that prevents expression of a complete set
of queen characters even if larvae are well-fed and not inhibited by the presence of a
fecund queen (Buschinger and Winter, 1975); (Hölldobler and Wilson, 1990). Similarly,
queen polymorphism in Leptothorax sp. A also has a genetic background (Heinze and
Buschinger, 1989).
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Recently, an effect of genotype on caste differentiation was reported in
Pogonomyrmex rugosus (Julian et al., 2002) and P. barbatus (Julian et al., 2002; Volny
and Gordon, 2002). Where these species are sympatric and apparently only there,
virtually all queens are homozygous and all workers are heterozygous in both species.
For new queens to be able to produce both queens and workers, they must mate with at
least two males, one of each haploid genotype (Julian et al., 2002). Whatever the origin
of this mating system in the zone of sympatry, it is clear that the function of a locus
involved in caste determination has been perturbed.
Fourth and finally, although no genetic basis for soldier induction in Pheidole has
been demonstrated, the system of constant production of a caste is intriguingly similar
to that of queen production Melipona in that it is constitutive.
Soldier production,
however, is homeostatically regulated, with production increasing when percentage of
adult soldiers drops and decreasing as proportion of adult soldiers increases. Even
when the colony is provided with ample protein and adult soldiers removed or other
social manipulations applied, only a limited proportion of larvae develop into soldiers
(Wheeler and Nijhout, 1984; Passera et al., 1996). It is conceivable that genetic factors
limit the responsiveness of some individuals.
Major landmarks in biology in the mid twentieth century
In the mid 20th century, major advances in biology came quickly. Landmarks
included: 1) development of an evolutionary synthesis that bridged genetics and the
biology of natural populations by T. Dobzhansky (Dobzhansky, 1937), R.A. Fisher
(Fisher, 1930), J.S. Huxley (Huxley, 1942), E. Mayr (Mayr, 1942), S. Wright (Provine,
1986) and others, 2) W.D. Hamilton’s suggestion that kin selection allows the evolution
of altruism in genetic models (Hamilton, 1964), 3) the chemical identification of
12
ecdysone (Butenand and Karlson, 1954) and juvenile hormone (Röller et al., 1967) and
an understanding of their physiological roles, and 4) the identification of DNA as the
molecule that transmitted traits (Avery et al., 1944) and a model that proposed how its
structure contained a genetic code (Watson and Crick, 1953).
The Modern Synthesis
As it became clear that gene theory alone, as developed by the early geneticists,
could not explain evolution, numerous
biologists interested in natural populations
began to build bridges to genetics.
The
result was what was called the Modern
Synthesis or the Evolutionary Synthesis
which brought together various aspects of
natural history and genetics, as indicated in
Figure 3. Unification of genetics and some
aspects of natural history
the figure on the right. An example of work with caste that fit in well within the new
framework was that of E.O. Wilson on the quantification of worker castes in ants and
thoughts on potential evolutionary transitions (Wilson, 1953). Note that developmental
biology was not included in the Modern Synthesis (Gilbert et al., 1996), and it is within
developmental biology that caste determination falls. The full integration of evolution,
development and genetics would not take place for another 50 years.
Genes and altruism
In the area of theoretical population genetics, W.D. Hamilton (Hamilton, 1964)
showed a theoretically robust genetical basis for altruism. If the success of genes
shared with relatives is included in total fitness, then even sterile individuals can have
fitness if their relatives are very successful. Helping behavior of sterile social insects
13
could assure their fitness in this way. Hamilton offered the simple inequality that the
ratio of cost and benefit incurred by helping must be less than the degree of relatedness
(C/B<r). The beautiful simplicity of this insight and its power to predict many features of
social structure in social insect colonies was dazzling. For decades, aspects of kin
selection, parental manipulation and mutualism were argued heatedly. The arguments
have now cooled down and most evolutionary biologists are comfortable with the
concept of kin selection operating in the context of parent-offspring conflict (Hölldobler
and Wilson, 1990). The theory, particularly in its simplest application to relatedness
asymmetries, has not proved to be all-sufficient. But, as a reading of Hamilton’s paper
will show, it was never meant to be a complete solution.
What implications does kin selection or selection at the colony level have for
understanding mechanisms of caste determination? Certainly, caste determination can
be subject to conflict, and conflict is obvious in the culling of queens by workers in
polygynous ants and Melipona bees. In order for actual conflict to occur , larvae must
have the ability to select their caste fate (Bourke and Ratnieks, 1999). An enhanced
genetic bias in Melipona bees could be viewed as a means for at least a proportion of
queens to achieve a higher level of self-determination (Ratnieks, 2001).
Queen inhibition acting before the point in development where nutrition is the
deciding factor can take away an individual’s ability to choose its caste (Wheeler, 1986).
Resolution of caste conflict by queen pheromones could indicate that queens have won
the conflict by physiological means, as in Bombus terrestris perhaps (Röseler, 1975).
But such a reduction in conflict could also indicate that selection is operating on a higher
level than the individual and represents a means of resolving conflict (Maynard Smith
and Szathmáry, 1995). Not all features of caste determination are conflict based. The
14
interests of all castes and the colony may be aligned regarding the responsiveness of
larvae to seasonal cues that preclude their choosing the queen developmental pathway
(e.g. Myrmica rubra, Plagiolepis pygmaea). The temporal relationship between effects
of physical cues, caste pheromones, and nutrition may be useful as a record of the
evolution of caste control, whatever the evolutionary drivers of those controls might
have been (Wheeler, 1994). If the nutritional switch is the core mechanism for choosing
between developmental pathways, the other cues can be considered modifying factors
that reflect caste conflict or colony level selection. In the case of Melipona bees, the
nutritional switch itself may have been tampered with by genetic modifications.
Identification of insect hormones and their physiological power
The discovery of juvenile hormone produced great excitement over the possibility
of pesticides that would specifically target pest species and leave non-targets
unharmed. Beyond the identification and characterization of the hormones(Butenand
and Karlson, 1954; Röller et al., 1967), Carroll Williams at Harvard University was one
of the leaders in the new age of insect physiology (Williams, 1956).
He used
photogenic subjects, Cecropia moths, in experiments that clearly showed the
morphogenetic power of juvenile hormone.
H.F. Nijhout was a graduate student at Harvard in the early 1970s, where he
worked with Carroll Williams and interacted with E.O. Wilson. Nijhout suggested to me
in 1978 that no one had looked at the effect of insect hormones on worker castes in
ants.
Others saw an opportunity for work in this area also. Juvenile hormones could
be purchased through Sigma Chemical and analogs were available through companies
such as Zoecon; and anyone who had a notion could apply juvenile hormone on
anything.
15
For example, in 1974, Donald Murphy, an undergraduate at the University of
Missouri, applied the analog Altosid to larvae of Pheidole dentata.
Treatment of the
brood resulted in an increase in the number of soldiers as well as soldier-queen
intermediates (Murphy, 1974). He wrote to E.O. Wilson to ask him his opinion of the
results. Murphy was planning to go to graduate school in Neurobiology and never
followed up his undergraduate work (Murphy correspondence courtesy of E.O. Wilson).
By 1978 in France, Luc Passera extended his work on caste determination in Pheidole
pallidula with applications of JH I. The treatments induced oogenesis in queens and
development of queen larvae but had no effect on soldier development (Passera and
Suzzoni, 1978). In 1981 and 1982, he fed colonies on prey dosed with a juvenile
hormone analog and did see an increase in soldiers production (Passera, 1985). In
Japan, Shigeru Ono also applied a juvenile hormone analogue to larvae of Pheidole
fervida and found a possible increase in soldier production and one soldier-queen
intercaste (Ono, 1981).
The results of my dissertation covered both the induction of soldier castes with
the JH analog methoprene (Wheeler and Nijhout, 1981b; Wheeler and Nijhout, 1983)
and reduction of methoprene effectiveness in the presence of atypically high proportions
of adult soldiers (Wheeler and Nijhout, 1984). As I compiled the accumulating literature
about the effects of juvenile hormones across the social Hymenoptera, the initial jumble
of effects resolved into a picture of JH having effects on caste anytime from
embryogenesis to the adult, depending on the taxon. JH usually seemed to be involved
when nutrition was the important determinant of caste (Wheeler, 1986).
Overall, the initial promise of juvenile hormone in enabling us to explore the
processes of caste determination has not been fulfilled (Gilbert et al., 2000). Other
16
insect hormones that fit into the standard peptide and steroid classes have yielded a
wealth of information to biochemical and molecular methods. But in the case of juvenile
hormone, we still don’t even know its mechanism of action (Gilbert et al., 2000).
A
breakthrough in understanding how JH works would be a major advance for research in
caste determination.
DNA, molecular biology, and the era of genomics
The discovery of the genetic code in the 1950s led to an enormous expansion in
research related to the central dogma that DNA makes RNA makes protein. Progress
was first made with bacteria and eventually with a few eukaryotic model systems.
Molecular biologists were often accused of the sort of arrogance and hubris that
characterized the earlier geneticists. The findings and the technology had no impact
studies on caste determination, since the available technology was not appropriate for
tackling development in outlaw (non-model) systems.
In the 1990’s, molecular biological methods crossed the threshold that made
them useful in examining the relationship between genes and phenotype in social
insects. The method called differential display (Liang and Pardee, 1992) was the first to
offer this ability, but further advances combined with expression arrays have proven
more informative (Diatchenko et al., 1996).
As with the simultaneous but multiple,
uncoordinated efforts to explore the effects of hormones on Pheidole ants, molecular
tools began to be applied to castes, with honey bees as the model of first choice.
In
1999, three papers were published reporting differential gene expression in honey bee
workers and queens. In March, Hepperle and Hartfelder identified two regulatory genes
using a differential display reverse transcription protocol (Hepperle and Hartfelder,
2001). In April 1999, Corona et al. reported 3 differentially expressed mitochondrial
17
genes (Corona et al., 1999) and in May, Evans and Wheeler reported 7 more (Evans
and Wheeler, 1999).
Evans and Wheeler followed up their first report with an
expression analysis of 158 unique ESTs across larval development. Results clearly
indicated that a substantial amount of differential gene expression accompanied caste
determination and early differentiation (Evans and Wheeler, 2000a). This has brought
us into a new era of exploration into caste determination and related phenomena in
insects (Evans and Wheeler, 2000b).
This new era includes both the tools molecular
biology now offers and a more holistic biology that is emerging through genomics (see
below).
Genomics and the Return of the Phenotype
The completion of the human genome surprised many with the result that our
species has only about 30,000 genes, merely twice that of the fruit fly. A reporter for
the Public Broadcasting System (Davies, 2001) wrote
After a decade of hype surrounding the Human Genome Project, punctuated
at regular intervals by gaudy headlines proclaiming the discovery of genes
for killer diseases and complex traits, this unexpected result led some
journalists to a stunning conclusion. The seesaw struggle between our
genes – nature—and the environment—nurture—had swung shortly in favor
of nurture. “We simply do not have enough genes for this idea of biological
determinism to be right,” asserted Craig Venter, president of Celera
Genomics, one of the two teams that cracked the human genome.
In some ways, it is surprising and perhaps sad that molecular geneticists are still
clinging to the idea of biological determinism.
As the field of genetics formed in the
early twentieth century, the importance of interaction between genes and their
environments was understood.
Johannsen, a Danish scientist, is given credit for
defining the concepts of genotype and phenotype (Johannsen, 1911). He wrote,
“I have proposed the terms “gene” and “genotype” and “phenotype” and
“biotype” to be used in the science of genetics…. Of course the phenotypes,
18
the reactions of the genotypical constituents, may under different conditions
exhibit all possible forms… Every student of genetics ought to know this.”
Unfortunately, in the remainder of the twentieth century, the importance of the
environment and its effects on gene function was widely under appreciated.
The
discovery of the structure of DNA and the formulation of the central dogma that DNA
makes RNA makes protein was enabled and then driven by scientists coming out of the
traditions of chemistry and physics. As James D. Watson wrote in his book The Double
Helix,
Science moves with the spirit of an adventure characterized both by youthful
arrogance and by the belief that the truth, once found, would be simple as
well as pretty (Watson, 1968).
It was as if genotype by environment interactions ruined the beautiful simplicity of the
double helix and central dogma and was best overlooked.
At the time the Modern Synthesis brought together genetics and evolution, a few
biologists were developing what are now called alternative syntheses that united
genetics,
evolutionary
biology,
AND
embryology.
Conrad
Hal
Waddington
(Waddington, 1953) and Ivan Ivanovich Schmalhausen (Schmalhausen, 1949) came
out of the European embryological tradition which was less focused on the gene. Both
argued for the importance of interactions between the cytoplasm and the nucleus as
well as interactions with the environment. Both developed similar concepts called, in
Waddington’s terms, canalization and genetic assimilation (Gilbert, 1994; Gilbert, 2000).
The works of Waddington, Schmalhausen and others were reconsidered in the
mid 1980’s as population geneticists and evolutionary biologists seriously tackled
phenotypic plasticity (Pigliucci, 2001). As an indication of interest in the subject, articles
accessed by searching the ISI Database on the linked terms phenotypic plasticity and
evolution jumped from 4 during 1986-1990 to 209 during 1991-1995.
19
Particularly
relevant to social insects studies, the polyphenisms of social insects were put into the
same picture as other polyphenisms, of which insects have a particularly diverse array
(Nijhout and Wheeler, 1982; Nijhout, 1994). West-Eberhard convincingly argued that
insect sociality had epigenetical origins (West-Eberhard, 1987) and phenotypic plasticity
in general was an important factor in the origins of biological diversity (West-Eberhard,
1989).
The complete sequencing of the genomes of various model organisms, including
8 eukaryotic ones to date, has led to the realization by many that gene function should
be considered only in the context of their cellular environments.
The cellular
environment can confer a considerable range of effects, as noted by Johannsen in
1911. And a glance at an ant colony confirms the extent of this range. The awakening
of biologists, especially geneticists and molecular biologists, to the importance of the
cellular environment can be seen in an explosion of productivity in such areas as
proteomics (Fields, 2001), evolutionary systems theory (Robert et al., 2001),
epigenetics and phenotypic evolution (Schlichting and Pigliucci, 1998) (Newman and
Müller, 2000; Pigliucci, 2001), and the renaissance of generalism (Simpson, 2001).
The title of one article promoting an organismic perspective declares “The gene is not
dead, merely orphaned and seeking a home” (Hall, 2001).
Relevance to studies on caste determination
The potential for studies on caste determination to answer fundamental
questions concerning the evolution of polyphenisms (Nijhout, 1999; Evans and Wheeler,
2000b) has been greatly enhanced by the development of molecular biological tools
that can be applied to minute amounts of tissue. In addition, the sequencing of the
20
honey bee genome, to begin soon, will allow the full arsenal of computational tools
available for genome analysis to be applied to a highly social insect.
At least as important, however, is the development of an intellectual climate that
appears to be facilitating an ultimate synthesis of biological thought.
As part of the
ultimate synthesis, biologists are realizing that if structures and functions are the results
of genes x environment interactions (Newman and Müller, 2000), then evolution, now
seen as a change in frequencies of particular gene x environment interactions, and the
resulting changes in populations can be due to changes in either the frequencies of
genes or environment.
Environments influencing gene action extend from within the nucleus, cells and
organism to the external environment.
Environmental factors include temperature,
photoperiod, diet, population density, presence of predators (Gilbert, 2001) and in the
case of social insects, factors arising from the social environment (Evans and Wheeler,
2000b).
Over a century ago, Darwin used castes in social insects to test his model of
evolution. Today, castes are still a prime test for any new biological synthesis that
views organisms as the result of interactions between genes and environments.
21
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