IS A BRYOPHYTE SPOROPHYTE A PARASITE?
I was reading Chapter 2-1 of your book on Bryophyte Ecology; at the end, the 'Summary' states that the
gametophyte supports a parasitic sprophyte. I believe this is a mistake that has caused long discussions among
teachers in our introductory courses. The growing sporophyte causes no damage to the gametophyte and once in the
open, it is photosynthetic. Clearly, there is a misconception of the relationship between the gametophyte and the
sporophyte. To me, it would be like saying that my fingers or hands are parasitic on my trunk! What do you think?
Claudio Delgadillo
In reading my Chapter 2-1 Summary, Claudio Delgadillo has raised the question of whether we can truly consider
the sporophyte of a bryophyte to be a parasite on the gametophyte. He argues that it would be like considering my
hand to be a parasite on my body. He further argues that the sporophyte causes no damage to the gametophyte and
that it is photosynthetic for part of its existence. Claudio's points are well founded, so I decided to explore it a bit on
the internet to see how others consider the relationship.
The first definition I found for parasite is: an animal or plant that lives in or on a host (another animal or plant); it
obtains nourishment from the host without benefiting or killing the host. I agree that the sporophyte is capable of
fixing photosynthate, but it also derives nourishment from the gametophyte, so I feel that part of the definition is
satisfied. Thus, the question becomes, is the sporophyte a different plant by virtue of being a different generation?
To follow the human analogy, is a fetus a parasite on its mother?
I would consider that the sporophyte does do the gametophyte harm, as discussed in literature cited in Chapter 5-9
of my bryophyte ecology book <http://www.bryoecol.mtu.edu/>. Its requirement for energy from the gametophyte
greatly reduces the biomass gain of the gametophyte compared to those gametophytes not supporting a sporophyte,
at least in the species studied.
At this point, it would seem one could argue either way. Convention becomes our guide, as language is a product
of human use. The University of British Columbia web site on bryophytes states "The sporophyte grows partly as a
parasite on the gametophyte." The University of Illinois Chicago website states "Sporophyte is a parasite." The
World of Biology Website states "In Seed Plants, the Gametophyte is a very small Parasite of the Sporophyte."
"The young sporophyte, as for ferns, develops initially as a parasite on the gametophyte." Similarly, in Brittanica,
"Among the gymnosperms the male gametophyte is much reduced and is a parasite on the sporophyte for only a
short time." The Berkeley Tree of Life website refers to algae: "Small sporophyte is dependent on a larger
dominant gametophyte." Clearly the concept of a plant being parasitic on its other generation is not unique to
bryophytes.
On the other hand, a German website, referring to bryophytes, states "If these both wouldn't belong to the same
species, the sporophyte could be called a parasite on the gametophyte."
This brings us full circle. Can we legitimately call a species a parasite on itself? It seems to me we have
broadened our use of the term to include such instances. We have a history of doing this, particularly in ecology,
where usage of the term plant strategy became very controversial, but it has stuck in the literature.
Janice Glime
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RESPONSES
I would vote no - the sporophyte is not a parasite on the gametophyte.
Why no- because it is part of the life cycle of the system, and the gametophyte clearly benefits from the interaction
(offspring production) . Although I suspect that the connected gametophytic tissue dies in the process. Clearly we
can ask what are the true effects of the interactions to both players (I am ignoring the male/sperm and spores) I
suspect we will need to carefully argue some of the possible +, - or 0 effects.
Using parasite sounds provocative and draws interest from others (etc the public/students). To think a fetus / seed
is a parasite on the parent is another way to look at parent/offspring interactions and can even be very useful in
tapping into theory from another field to understand this relationship/interaction.
Another interesting case are the males of some deep sea fish that are very small and are parasitic on females :) .
(these are clearly different individuals)
I guess diversity does interesting things to definitions
take care
Nicholas
-D. Nicholas Mc Letchie
Ph 859 257 6786
fax 859 257 1717
http://www.as.uky.edu/biology/faculty/mcletchie/
Department of Biology
101 Morgan Building
University of Kentucky
Lexington, KY 40506
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Janice,
I say yes. It is definitely not the same as saying my hand is parasitic on the rest of my body. I do not believe that
there is a similar situation in animals for a comparison.
Sincerely,
Keith Bowman
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No.
If we say yes, I think that we're abusing the word "parasite''. I agree with
what the German website says, since both the gametophyte and sporophyte belong
to the same species, we can't call the sporophyte a parasite.
As to the definition "parasite-" it obtains nourishment from the host
without benefiting or killing the host'', I'll say that the sporophyte does
benefit the host. Without a successful reproduction system, there will be no
more gametophytes. That's how life is.
Hua
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This is a very interesting question which brought back many memories of my first academic job interview at the
University of Florida. My interview seminar dealt with carbohydrate metabolism in source and sink tissues. While
the main part of my talk dealt with carbohydrate metabolism in potatoes, I had recently done work on carbohydrate
metabolism in the gametophyte and sporophyte of several moss species. I was asked the question, "If mosses lack
vascular tissue how could you talk about a source and a sink." I know this is a little off your question but it got me to
thinking about the relationship between the sporophyte and gametophyte. I don't think I would class the generations
as being parasitic since each generation derives a benefit from the other. I think the association would be more
mutualistic than parasitic. The sporophyte generation would get the more immediate benefit from the photosynthate
from the gametophyte but, the moss as a whole would benefit due t! o genetic
Cyndy Galloway
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No. How about "matrotrophophyte" (noun) or "matrotrophophytic" (adj.)
from matrotrophy. Matrotophy has common usage in zoology and botany.
Why give the sporophyte the negative connotation as being a parasite?
I see no value in this other than to convey the concept of partial
dependence on the gametophyte. It is not a parasite anymore than a
human fetus as you point out.
Thank you for the opportunity to offer an opinion.
Paul G. Davison
UNA Box 5232
Department of Biology
University of North Alabama
Florence, AL 35632
Phone (256) 765-4434
Fax (256) 765-4430
http://www2.una.edu/pdavis/
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From my point of view one generation of a species can not be the
parasite of the other generation.
Without the sporophyte there would be not sexual reproduction of mosses.
As a consequence, this would harm the evolution of mosses.
So, there are two more biological arguments against this concept:
1. Mosses benefit from the alteration of generations in terms of
evolution and dispersal.
2. Parasitism is an option (a thread) to the host without benefit for
the host. Alteration of generations is far more than an option.
Warm regards,
Ralf.
-Ralf Reski
Professor
Dean Faculty of Biology
Head Plant Biotechnology
University of Freiburg
Schaenzlestrasse 1
D-79104 Freiburg
Germany
Fon: +49 761 203-6969
Fax: +49 761 203-6967
ralf.reski@biologie.uni-freiburg.de
http://www.plant-biotech.net/
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Yes, but I'm open to alternate language and look forward to the responses
you get! A fetus hormonally mines its mother of calcium and nitrogen, so to
me it is a very well-adapted parasiste...even using specialized globins for
oxygen transfer.
FWIW: I never use "fertilization" when I mean "syngamy" because of the
obvious inaccuracy and confusing interpretation of the former...for
botanists "fertilization" often means application of minerals, compost, etc.
rather than union of gametes...and females are fertile whether male gametes
are available or not...so just as NASA forced physiologists to stop using
geotropism when they mean gravitropism...we do need to make sure our terms
accurate.
An interesting thread!
ross
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They are separate organisms, despite their genetic relationship.
-__________________________________
Martha E. Cook
Associate Professor of Botany
Illinois State University
Department of Biological Sciences
Campus Box 4120
Normal, IL 61790-4120 USA
Phone: 309-438-8549
Fax: 309-438-3722
E-mail: mecook1@ilstu.edu
http://www.bio.ilstu.edu/cook/
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Parasitism is a type of relationship that involves two individuals of different species (from Wikipedia: “two
organisms which are phylogenetically unrelated”). Thus, I think it is not well used to describe the relationship
between gametophyte and sporophyte in bryophytes, because in this case there is one only species (and maybe one
only individual, or how many individuals are there in a fructified gametophore?). As an exception, parasitism can
occur within the same species, as in the case of brood parasitism: brood parasites lay their eggs in the nests of hosts
of the same or different species. Intraspecific brood parasitism has been reported for a range of insect, bird and fish
species.
In addition, in parasitism one individual (the parasit) benefits and the other (the host) is harmed. In bryophytes,
gametophyte growth may be negatively affected by sporophyte production, but spore formation may contribute to
genetic diversity and thus to environmental adaptability and ecological success. Thus, it is debatable if the supposed
“host” (gametophyte) is harmed or benefited or both.
I think that the term “parasitism” is being successful because of a simple question of fashion, in the line of similar
“sensationalist” terms such as “plant strategies” or “invasive alien plants”.
There is a term (auxotrophy) that sometimes has been used to describe the relationship between gametophyte and
sporophyte in bryophytes. From Wikipedia:
“Auxotrophy is the inability of an organism to synthesize a particular organic compound required for its growth (as
defined by IUPAC). An auxotroph is an organism that displays this characteristic; auxotrophic is the corresponding
adjective. Auxotrophy is the opposite of prototrophy.
In genetics, a strain is said to be auxotrophic if it carries a mutation that renders it unable to synthesize an essential
compound. For example a yeast mutant in which a gene of the uracil synthesis pathway is inactivated is a uracil
auxotroph. Such a strain is unable to synthesize uracil and will only be able to grow if uracil can be taken up from
the environment. This is the opposite of a uracil prototroph, or in this case a wild-type strain, which can still grow in
the absence of uracil. Auxotrophic genetic markers are often used in molecular genetics.
Researchers have used strains of E. coli auxotrophic for specific amino acids to introduce non-natural amino acid
analogues into proteins. For instance cells auxotrophic for the amino acid phenylalanine can be grown in media
supplemented with an analogue such as para-azido phenylalanine. Aminoacyl tRNA synthetases recogonize the
analogue and catalyze its binding to a tRNA which subsequently transfers the amino acid (non natural in this case)
to a growing polypeptide chain during protein translation.
It is important to remember that many living things, including humans, are auxotrophic for large classes of
compounds required for growth and must obtain these compounds through diet (see vitamin, essential amino acid,
essential fatty acid).”
The term “auxotrophy” does not fit exactly with the relationship between gametophyte and sporophyte in
bryophytes, but it may help. Probably this relationship is so unusual in nature that we need to invent a new term.
With best regards,
Javier
-**********************************
Dr. Javier Martinez-Abaigar
Universidad de La Rioja
Complejo Cientifico-Tecnologico
Avda. Madre de Dios 51
26006 Logroño (La Rioja)
(España, Spain)
Tel.: 34+941+299754
Fax: 34+941+299721
javier.martinez@unirioja.es
http://www.unirioja.es/ecophys/
*********************************
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Yes, I consider the sporophyte parasitic on the gametophyte. Matritrophy is key
to sporophyte survival, but the word is too loaded. The sporophyte is
dependent, like a child is on its mother, for as a child can breath so too may
a sporophyte photosythesise (perhaps even using atmospheric carbon via its
stomata, in the case of mosses).
C. C. C. Chater, UK
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This brings to mind that I once read something about excised hornwort
sporophytes growing in vitro--can't remember details or the source. I once
found a population of hornworts where rain runoff had completely covered the
gametophytes, but the sporophytes (many of them several cm in length)
appeared quite healthy and growing vigorously. There were many dozens of
them, growing densely enough that the population resembled a patch of
strange graminoids. The gametophytes I retrieved from the sandy soil were
black and shriveled so far as I could see with a handlens. I did not examine
any of them in the laboratory, but it did seem that the gametophytes were
serving as little more than perhaps a sort of wick between sporophyte and
soil solution.
I am wondering if there have been any observations of further maturation in
the relatively large chlorophyllose young sporophytes of certain mosses if
they are excised and maintained in vitro?
Dan Marsh
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About the question, I think that the sporophyte is not parasite of gametophyte, first is the same species although a
different generation. The development of sporophyte is necessary to complete the life cycle, so if the sporophyte
complete its life and produce a lot of spores it will be good for the species ( including gametophyte).
Is the root of a tree a parasite of the photosynthetic part of the tree? or Is the female gametophyte of angiosperms
parasitic of sporophyte ? I think no.
sincerely
Felisa
M. Felisa Puche Pinazo
Departamento de Botanica
Facultad de Ciencias Biologicas
tel. 96 354 4633
Universitat de Valencia
Dr.Moliner n 50, 46100- Burjassot
Valencia. Spain
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Thanks you very much of the interesting discussion on the use of the term "Parasite".
I personally agree that the sporophyte is, at least partially, a parasite of the gametophyte. In human body, hands are
not parasites as they are part of the body, whereas in plants, gametophytes and sporophytes are completely different
entities. As one is haploid other is diploid, the genetic make up is at least partly different. Consequently, I would
consider animal fetus a parasite on its mother. I think that is a evolutionary strategy to nurse the next generation.
Animals that hunt and eat its own species are still consider carnivores (although it is a special kind of carnivory call
cannibalism), why can't we call a species a parasite when it get its nutrition from another individual of the same
species? Unless someone wants to invent a new term for this special kind of parasitism of plants.
Best wishes
Boon-Chuan Ho
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*German website, referring to bryophytes, states "If
these both wouldn't belong to the same species, the sporophyte could be
called a parasite on the gametophyte."*
The question is a bit semantic and perhaps even philosophical, but I think
the above statement is correct. How can a sporophyte be defined as a
parasite of a gametophyte, if the sporophyte's whole existence only aims at
producing more gametophytes like the one it is sitting on? True, the new
gametophytes will not be genetically identical with the original one, but
that is not the point here, particularly since the gametophytes of many if
not most bryophytes are capable of producing clones of themselves if and
when they "have to". I think the term parasite is wrongly applied here.
Best regards,
Johannes
-Dr. Johannes Enroth
PhD, University Lecturer, Bryologist
Dept. of Biological and Environmental Sciences
P.O. Box 65
FIN-00014 University of Helsinki
Finland
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Dependant vs parasitic?
Is this distinction fouynded on whether the sporophyte actively enforces a
regieme of growth and development onto the gametophyte, the result of
which is the nutritional interface and protective structures produced by
gametophyte.
Does the gametophyte (as does any mother) actively facilitate the growth
and development of the sporophyte? Does a chemical stimulus from the
sporopyte initiating a signal transduction cascade (or whatever transmits
that signal) in the sporophyte constitute the same kind of enforcement as
a tick sticking its mouthparts into you?
I would argue that the relationship between sporophyte and gametophyte
stands in sharp contrast to the relationship between host and parasite,
regardless of nutritional similarities in both types of relationship.
regards
Matt
_______________________________________
Matt Renner
PhD candidate
The Wardle Lab
School of Biological Sciences
Heydon Laurence Building A08
University of Sydney, NSW 2006
(02)9351 5118
041 585 2205
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There is a whole generation of my high school students who learned a rhythmic "choral reading" of a short poem
that went like this:
In (pause) mosses,
the sporo-phyte
is a para-site
on the fe-male
gameto- phyte.
I stood at the front and waved my arms around like a choir director as we said this over and over. They were very
young high school sophomores, and I did whatever I could to keep them focussed and have a little fun along the
way.
They were also quite attentive after I told them that mosses and ferns can only do it in the rain....smile.
Sadly, with the advent of standardized state tests and therefore, standardized curriculum topics, there is no longer
time to teach a survey of either the plant or animal kingdoms. I guess when we retire we're allowed to reminisce
about the good old days.
Maggie Ray
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Perhaps the term hemi-parasite may better sum up the situation in most,
but not all, bryophytes. I think the situation in mosses may be more or
less unique.
In Marchantia, or other marchantioid hepatics, the sporophyte undergoes
much of its development without the apparent need for photosynthesis.
Riccia is perhaps the extreme case but even in Asterella, Marchantia,
Lunularia, Reboulia, the carpocephalum contains photosynthetic tissue
but it may be of use (to the sporophyte itself) only in the final stages
where the sporophytes are elevated above the thallus.
Jungermanniales do not, as far as I am aware, have photosynthetic tissue
in their sporophyte.
I wouldn't consider my arm, leg, head even, parasitic on the rest of me
because each part derives all it needs (I think!) from the whole.
If one looks at the case of Buxbaumia, what do we call the maturing
sporophyte - which is very much larger and more conspicuous than the
gametophyte (which, I hasten to add, I have never actually examined or
seen).
If one looks at Ephemeropsis, and perhaps to a slightly lesser extent
Viridivellus, where the gametophyte is about the size of the sporophyte
or may, because it is like a protonemal felt, be the dominant part, we
have perhaps a 50:50 situation.
In many instances the moss sporophyte is the lesser party. But, if we
have to draw analogies because our minds like to have comparisons or
places in which to put things, I think the instance of the animal foetus
is probably the equivalent of the situation in a bryophyte - no, moss sporophyte.
In hepatics, the sporophyte perhaps comes nearer to a truly parasitic
relationship - but this then gets back to the philosophical argument
about the possibility of being parasitic on oneself.
The hornwort sporophyte is perhaps nearest to independent existence, but
it depends on sperm (possibly foreign in the sense of being from another
plant) being subsumed in nuclear fusion by the gametophyte egg cell to
form the zygote.
Perhaps not entirely unrelated to this discussion is the thesis once put
forward that the chloroplast was originally a foreign organisms captured
by another and subsumed in the dominant partner's make up. There is
chloroplast DNA after all. Perhaps the molecular people can answer the
question - how different is the chloroplast DNA to, nuclear DNA or
ribosomal DNA?
Regards,
Rod Seppelt.
Prof. Rod Seppelt,
Australian Antarctic Division
Channel Highway,
Kingston, Tasmania 7050,
Australia
ph: +61 (03) 62 323 438
FAX: +61 (03) 62 323 449
email: rod.seppelt@aad.gov.au
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Why not a hemi-parasite?
I wouldn't consider my arm, leg, head even, parasitic on the rest of me because each part derives all it needs (I
think!) from the whole.
If one looks at the case of Buxbaumia, what do we call the maturing sporophyte - which is very much larger and
more conspicuous than the gametophyte (which, I hasten to add, I have never actually examined or seen).
If one looks at Ephemeropsis, and perhaps to a slightly lesser extent Viridivellus, where the gametophyte is about
the size of the sporophyte or may, because it is like a protonemal felt, be the dominant part, we have perhaps a 50:50
situation.
In many instances the moss sporophyte is the lesser party. But, if we have to draw analogies because our minds like
to have comparisons or places in which to put things, I think the instance of the animal foetus is probably the
equivalent of the situation in a bryophyte - no, moss - sporophyte.
In hepatics, the sporophyte perhaps comes nearer to a truly parasitic relationship - but this then gets back to the
philosophical argument about the possibility of being parasitic on oneself.
The hornwort sporophyte is perhaps nearest to independent existence, but it depends on sperm (possibly foreign in
the sense of being from another plant) being subsumed in nuclear fusion by the gametophyte egg cell to form the
zygote.
Prof. Rod Seppelt,
Australian Antarctic Division
Channel Highway,
Kingston, Tasmania 7050,
Australia
ph: +61 (03) 62 323 438
FAX: +61 (03) 62 323 449
email: rod.seppelt@aad.gov.au
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Parasitism? I would say: no.
The problem of course lies in the definition of 'parasitism'. The one given here uses the 'plant or animal' (as if
nothing else would exist, and I think here you can clearly see how traditional and conventional this approach is) as
the unity. May be, we should think about the sporophyte and the gemetophyte as expressions of one and the same
genome, and, in a sense, parts of a same 'organism'?
Hans de Bruijn
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Its an interesting question. I checked the mening of parasitism by wikipedia: http://en.wikipedia.org/wiki/Parasite
I think for bryophytes this word can not be used because: it should be between phylogentically different individuals
and the parasite must cause harm to the host:
("It is important to note that "benefit" and "harm" in the definition of parasitism apply to lineages, not individuals.
Thus, if an organism becomes physically stronger as a result of infection but loses reproductive capabilities (as
results from some flatworm infections of snails), that organism is harmed in an evolutionary sense and is thus
parasitized.")
Since the relationship between the gametophyte and sporophyte is a necessity for reproduction (well...hm.
depending on if the species use sexual or vegetative reproduction) it is an evolutionary benefit.
Perhaps its time to come up with a new term for the special relationship between gametophytes and sporophytes in
bryophytes (a relationship that causes temporary harm to the host but is an evolutionary benefit in the longer term).
But on the other hand: it is very similar to other cases as you already mentioned. Cases that are not considered to be
parasitic.
So, I do not know. Unfortunately, the world is usually much more complicated that systematist and language allows
it to be.
Annika Jagerbrand
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Perhaps the term "hemiparasitic" could be better appropriate?
René Schumacker
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This is interesting but I think you have already answered the
question with what you presented. Maybe I'll make a misstep here, so
anyone correct me if I do. Since the gametophyte always gets the
benefit of sexual reproduction from the sporophyte, and since the
gametophyte produces the cells that lead to the creation of a
sporophyte, I simply can't see how the term "parasitic" applies to a
sporophyte. In essence, I don't see how one part of an organism is
parasitic on another part. As mentioned before, a fetus (imperfect
analogy since that is more like asking if a spore is parasitic on the
gametophyte - both are the same generation, and still, as you well
know the human placenta is not of the opposite generation from the
mother so it is even a weaker example than a sporophytic placenta) is
not a parasite and neither is a tumor. A tumor is also an imperfect
analogy, but closer to a parasite in my mind in that there is no
reproductive benefit to the host (at least in most cases). However,
the tumor is NOT another organism and therefore, I would argue that
it cannot be considered a parasite. I understand the desire to use
parasitic because it is provocative, but I think the term "dependent"
is appropriate and that we should be clear to make this distinction.
If the concern is how to characterize sporophytes that can provide
some photosynthate for themselves, I would use the term
"semi-dependent". As for the the chloroplast being enslaved by the
cell after endosymbiosis, one should realize that it is a two-way
street benefiting both former organisms - a real mutualism. To
partially answer Rod's question about differences in DNA, I believe
the G:C content is quite different, but more fundamentally (in my
mind) the chloroplast genome is a circuluar plasmid (just like
bacteria) rather than a set of chromosomes (like eukaryotes) and the
promoter structure is very different between nuclear and chloroplast
genomes. I couldn't be more convinced that the chloroplast is
essentially a bacterium within a eukaryote, but I won't go into all
the various sorts of evidence.
-Dave Hanson
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I believe that the term parasite reflects the mode of life of a species in relation to a different species. This is a species
fitness issue having population effects "In the population interaction called parasitism, one population (the parasite)
benefits, and the other (the host) is harmed by the relationship."
Because the plant sporophyte is just a different life phase of the same species as the gametophyte, their relationship
is no more parasitic than that of a eutherian mammal's fetus. I both cases (and in the florideophycean red algae and
cases of viviparous non-mammals) the fitness of the species is improved by maternal provision of nutrients to the
young. I call this "plant matrotrophy," a term first used in discussions of viviparity in animals.
The classic experiments of Adrian Browning and Brian Gunning demonstrated experimentally that matrotrophy
improves fitness in Funaria hygrometrica, a result that can reasonably be extrapolated to all embryophytes (in the
absence of evidence to the contrary).
I would say that the plant sporophyte is matrotrophic on the gametophyte for at least some period of time during
early embryo development.
Linda Graham
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It sounds like a tricky question if the sporophyte is parasitic or not, in
bryophytes. I think Ros Seppelt gave a nice introdution to the differences and
problems involved, thank you. In any case, I also would exclude parts of the
same organisms (and same generation) in the definition of parasitic.
Regarding chloroplast, generally three things are indicated as supporting an
endosymbiotic origin of it, a) a double membrane, b) and more interestingly, a
DNA which is of the prokaryotic type (no istones), and, c) ribosmes of the
bacterial type (different from those of eukaryotic cells), even more
interestingly, the same reasons apply to mitochondria too.
-Dr Carmine Colacino
Lab. di Briologia - Dip. di Biologia DBAF
Universita' della Basilicata
Viale dell'Ateneo Lucano, 10
Potenza, Italy
Ph. +39-0971-206234; Fax +39-0971-206233
Lab.+39-0971/205346: Mob.+39-329-3178399
Alt. e-mail: colacino@bryology.eu
http://www.unibas.it/utenti/colacino/
http://www.bryology.eu/
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The assertion of a parasitic sporophyte assumes that there is no benefit for
the gametophyte from the sporophyte. If one is to accept one of the basic
tenets of evolution...the primary goal is the survival of the DNA into the
future..., then the question seems to answer itself. The sporophyte is
preserving the genetic code of the gametophyte by producing spores. The
spores can survive inclement conditions, or even be transported in ways the
gametophyte cannot.
Obviously, bryophytes, with their infinite variety of asexual reproductive
strategies, do not rely on sexually produced offspring. However, the fertile
gametophyte spends a great deal of energy creating this "parasite".
Extending the human metaphor, the "parasite" child is not only an energy
drain in utero, but also for the first 15-20 years of life! This is an
awesome investment of energy. It is obvious that the species, and thus the
DNA would perish without offspring, and is thus clearly beneficial to the
parent.
So parasite is at best an inaccurate word for this relationship. Partner
seems a better place to start if one is convinced that the two generations
constitute a separate being.
Ken Kellman
9870 Brookside Ave
Ben Lomond, CA 95005
kkellman@sbcglobal.net
(831) 336-8548
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On "parasitic", I tend to think that this is a bit harsh to apply to moss sporophytes, since the sporophyte represents
offspring, and the gametophyte allocates nutrients toward their maturation, actively mobilizing their transfer as I'm
sure you are aware through an elaborate system of coevolved membranes at the sporophyte/gametophyte juncture.
When stressed, I'm finding that the maternal gametophyte jettisons the sporophyte to preserve herself, and this is to
be expected in perennial species, which then live to reproduce another set of offspring hopefully. Clearly, the two
generations signal each other, and the fitness of one is intertwined with the fitness of the other. I'm reminded of the
stimulation of postfertilization perichaetial leaves and the function they may confer in protecting the young embryos
of many pleurocarpous mosses (a phenomenon not expected in a parasitic situation).
As for an alternative term, would your suggestion of "hemiparasitic" work? To me the analogy is more to a fruit on
a tree. Maybe anything dealing with "parasitic" really connotes too negative for what is going on, however at the
moment I have nothing to put forward, perhaps something will come to me. When I think about the generations and
watch them grow, I tend to think of a single entity with two organs, rather than two separate entities vying for
survival.
Lloyd Stark
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Yes, I think that you have to consider the sporophyte as a parasite on the gametophyte. Considering Claudio's points
I would say that the situation is different to the hand and the body because the sporophyte is genetically distinct
from the gametophyte. Also, nobody doubts that mistletoe (Viscum album) is parasitic on the host tree, but it
appears to do the tree little or no damage and does also photosynthesise.
I think that what makes us feel uncomfortable is that the sporophyte is of some benefit to the gametophyte in that it
does disperse its genes. Perhaps we could say that the sporophyte is a physiological parasite but a genetic symbiote,
or is that too much of a mouthful?
Jonathan Sleath
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I shouldn't put the idea into university heads, but as an undergraduate at Queen's University in Kingston, Ontario in
the 1960's I had to undergo a final oral (as well as written) examination. So the question of whether or not the
sporophyte is parasitic would be a wonderful question for a student. It is entirely open-ended and would certainly
tell if the student could think rationally and quickly on his or her feet. One question I was asked during my oral was
whether or not viruses were alive. Nasty stuff. About the sporophyte - I'll leave that debate to those more
knowledgeable than I! Allan Aubin