CORRESPONDENCE
be more neutral or slightly deleterious mutations
than mutations that are immediately lethal, not
repairing DNA at all would lead to a much faster
accumulation of such slightly deleterious
mutations, and this would be disadvantageous.
(3) According to the ‘DNA repair hypothesis’7,
the primary function of sex is DNA repair and
complementation
of mutations; the genetical
variability is then only a by-product of this.
Outcrossing during recombination provides the
organism with an undamaged template for repair
processes, without the cost of permanently
retaining a redundant copy of the whole genome.
Asexually reproducing organisms thus have
less-efficient DNA repair systems, as only the
‘hypercycle model’8 or mitotic recombination can
be used for DNA repair in such lineages. The last
system, especially, is known to be about 1000
times less frequent than meiotic recombination9
and results in an increase of homozygosity in
further generations as sister chromatids are used
as repair templateslo.
However, strong evidence
for the ‘DNA repair hypothesis’ and its link to
sexual reproduction could be found neither in
transforming bacterial1 nor in modelling
evolutionary processes10,12.
Furthermore, another theoretical DNA repair
mechanism, the ‘prudent reparator’, which avoids
recombination during DNA repair and might
additionally contribute to overcoming MOller’s
ratchetlo, could explain how an efficient repair
system in asexuals will work without
recombination.
To sum up, there are insufficient arguments to
corroborate the hypothesis of deficient DNA repair
in persistent asexual lineages.
Isabelle Schoen
Dept of Biology, University of Leeds,
Woodhouse Road, Leeds, UK LS2 9JT
Koen Martens
Royal Belgian Institute of Natural
Sciences, Freshwater Biology,
Vautierstraat 29, 1040 Brussels, Belgium
Valeria Rossi
Dept of Environmental
Sciences,
University of Parma, Viale delle Scienze,
43100 Parma, Italy
References
Judson, O.P. and Normark, B.B. (1996) Trends
Ecol. Evol. 11, 41-46
Gabriel, W., Lynch, M. and Biirger, R. (1993)
Evolution 47,1744-1757
Marx, J. (1994) Science 266,728-730
Friedberg, E.C. et a/. (1995) Phi/OS. Trans. f?. Sot.
London Ser. 8 347,63-68
Modrich, P. (1995) Phi/OS. Trans. R. Sot. London
Ser. B 347,89-95
Seeberg, E., Eide, L. and Bjoras, M. (1995)
Trends Biochem. Sci. 20,391-396
Bernstein, H. et al. (1985) Science 229,
1277-1281
Eigen, M. and Schuster, P. (1979) The
Hypercycle,a Principle of Natural
Se/f-organization, Springer-Verlag
Orr-Weaver, T.L. and Stostack, J.W. (1985)
Microbial. Res. 1, 33-58
TREE vol.
I I, no.
7 July
1996
10 Szathm&y, E. and Kaver, S. (1991) Genet. Res.
Camb. 58,157-165
11 Mongold. J.A. (1992) Genetics 132, 893-898
12 Long, A. and Michod, R.E. (1995) Theor. Popu/.
Biol. 47, 18-55
Reply from O.P. Judson
and B.B. Normark
Ancient asexuals remain scandalous: reports of
their existence continue to incite alarm and
disbelief. As in all scandals, there may turn out to
be a respectable explanation. Little and Hebert
expect that the scandal of ancient asexuals will
blow over when most putative cases are exposed
(with better sampling) as impostors and the rest
are found (with better genetics) to be engaging in
covert sex (see Ref. 1). We agree with Little and
Hebert that claims of ancient asexuality should
not be taken at face value; indeed we included a
table of weakened or refuted claims specifically
to make this point. But we do not think that claims
of ancient asexuality should be dismissed a priori
as incredible. To date, putative ancient asexuals
have recieved more scepticism than study. Both
are needed.
Little and Hebert contribute some useful ideas
for studying putative ancient asexuals, in particular
their suggestion of screening for null alleles and
their focus on the diversity of genetic systems we
termed ‘asexual’. (Of course, if they are correct
that automixis promotes greater longevity that
apomixis, the usefulness of both their null allele
approach and the Meselson method will be
correspondingly limited.) However, they do make
one serious error. In no case did we rely on
studies that dated asexual lineages based on the
extent of taxonomic or genetic divergence from
their’ nearest sexual relative. Rather, every date
that we attribute to molecular data is based on
divergence within putative asexual clades.
Schoen et a/. agree with Little and Hebert that
Butlin and Griffiths’2 date for the most recent
sexuality in the darwinulid ostracods (which we
cited) is wrong. The real date, according to Schoen
et al., is the Permian; according to Little and
Hebert, it is the Recent. The males mentioned by
Little and Hebet? are indeed intriguing, but the
reports were made over a century ago and have
not been reconfirmed. Moreover, the most serious
recent student of Datwinula, LG. Sohn, regards
them as ‘dubious’3.
Schoen et al. are correct that loss of DNA
repair pathways in asexual lineages would not
be unconditionally advantageous. According to
the hypothesis of Gabriel et a/.4, the only repair
processes that should be lost in asexuals are
those that tend to transform lethal or highly
deleterious forms of DNA damage into less
deleterious mutations. A number of known repair
pathways, such as nucleotide excision-repairs,
have this property. One possible example of the
predicted adaptive loss of this kind of repair
pathway is the loss of the recFgene by obligately
clonal Buchnerae.
Let the claims of asexual antiquity stand or fall
with data, not prejudice. If they stand, the
mechanisms by which these organisms overcome
problems such as Miiller’s ratchet or attacks from
parasites will be of great interest. For instance,
one mechanism that would permit both problems
to be overcome is RNA editing - a process by
which information contained in a gene is altered in
the RNA template. Although it has only recently
been discovered’, it has been found in
mitochondria, chloroplasts and cell nuclei; it has
been reported in organisms from trypanosomes to
human+. We predict that ancient asexuals will
turn out to exist and that they will rely on devices
like this in order to respond flexibly to their
environments despite their lack of sex.
We thank N. Moran and R. Michod for helpful
discussions.
Olivia P. Judson
Benjamin B. Nonuark
Dept of Zoology, University of Oxford,
South Parks Road, Oxford, UK OX1 3PS
References
Hurst, L.D., Hamilton, W.D. and Ladle, R.J.
(1992) Trends Ecol. ho/. 7, 144-145
Butlin, R.K. and Griffiths, H.I. (1993) Nature364,
680
Sohn, LG. (1988) Proc. Biol. Sot. Wash. 101,
817-824
Gabriel, W., Lynch, M. and BBrger, R. (1993)
Evolution 47, 1744-1757
Hoeumakers, J.H.J. and Lehmann, A.R. (1994) in
Methods to Assess DNA Damage and Repair:
lnterspecies Comparisons (Tardiff, R.G.,
Lohman, P.H.M. and Wogan, G.N., eds),
pp. 57-82, John Wiley
Moran, N. Proc. Nat/ Acad. Sci. USA (in press)
Benne, R. (1988) EMBO J. 7,2509-2514
Scott, J. (1995) Cell 81,833-836
Valuing the Earth’s
resources
The article by Graciela Chichilniskyi on the
economic value of the Earth’s resources was
informative, insightful and generally wise.
However, I would like to call attention to a couple
of misleading notions that emerge frequently in
discussions of developed versus undeveloped
countries and are repeated in this article. First is
the unfortunate terminology of ‘the North-South
contrast’. This language is inaccurate and does a
disservice to those northern parts of the world that
are every bit as underdeveloped as much of the
tropics. If, for example, Chichilnisky and others
promoting the North-South dichotomy were to visit
the villages of native peoples in northern Canada,
I believe they would often find symptoms of poverty
and cultural disintegration just as severe as in
comparable villages of Latin America. This is no
minor quibble: patterns of growth, development,
and environmental degradation are extremely
heterogeneous within and among nations,
continents and hemispheres; a simple latitudinal
or North-South gradient is a fiction.
Another quibble I have is over Chichilnisky’s
interpretation of the relationship between the
human population and environmental degradation.
In one place (Box 1) she states that ‘[slince the
North has the lowest population growth, despite
0 1996, Elsevier Science Ltd
297