An interior designer once remarked that the two most
difficult things to get right in life are sex and
lighting. Sex, certainly, is a complicated business: so
much so that you would think most animals would find it
simpler to reproduce asexually-that is, by making clones
of themselves. But asexual reproduction in animals is
actually quite rare. Why?
Patricia J. Moore and colleagues of the University of
Manchester, UK, have been exploring one answer to this
question: once sex has evolved, they suggest, reverting
to an asexual or clonal mode of reproduction is
difficult.
Conventional wisdom has it that sex evolved because
of its manifest advantages, such as the maintenance of
genetic variation. Sex may have evolved, however,
because of the disadvantages of asexual reproduction.
That is, the benefits of asexual reproduction – not
having to dilute your genes with those of a mate – were
outweighed by physiological or genetic problems.
Getting a grip on what these problems might have been
is complicated by the fact that it is pointless
comparing animals-human beings, for instance-that
usually reproduce sexually with animals, such as aphids,
that regularly reproduce asexually. The reproductive
systems of these animals differ so much that the reasons
why some reproduce sexually and others asexually are
hard to fathom. More informative would be the study of
an animal that can reproduce both sexually and
asexually, depending on the circumstances.
As
they report in a new journal, Evolution and
Development1,
Moore and colleagues have found the very thing in the
laboratory cockroach, Nauphoeta cinerea. These
animals usually reproduce sexually. But in times of
crisis when males are scarce, females can reproduce by a
process known as 'parthenogenesis'. They can produce
offspring-all female-with no help from males.
This sounds like an evolutionary dream ticket, not to
mention the apotheosis of the feminist ideal. But in the
small print we find that only four in ten female
cockroaches can take advantage of this facility. The
resumption of asexual reproduction in the cockroach is
easily snagged on a 'developmental constraint'-sexual
reproduction. Once it has evolved, sexual reproduction
is indeed hard to evade.
For a female, there is a huge gulf between producing
unfertilized eggs that cannot proceed further without
fertilization, and eggs that can be coaxed into becoming
embryos without male participation. Technically, the
process revolves around 'meiosis', the rearrangement of
genetic material that takes place during the production
of sperm or egg cells.
Normally, an egg cell has half the adult number of
chromosomes, so that when it meets a sperm cell (also
with half the normal complement) the full chromosome
number is restored. Meiosis is the two-stage process of
cell division that produces sperm and egg cells, during
which this halving takes place. It is a tricky business
and mistakes can happen-egg and sperm cells may be
created with more or fewer than the regulation half-dose
of chromosomes.
In female cockroaches, it seems, meiosis can go wrong
so that egg cells are produced with a full set of
chromosomes. In certain circumstances these can develop
into embryos. But this scenario is doubly unlikely.
First, it requires a particular set of mistakes to
happen during meiosis that leads to precisely the
adult number of chromosomes. Second, it depends upon
this abnormal egg cell getting the chance to develop
further.
In other words, once sexual reproduction has evolved,
returning to asexual reproduction requires several
unlikely things to happen at once. This explains why
even female cockroaches, who are theoretically capable
of making the switch, do not seem to do so as often as
they might.
In which case, asexual reproduction is more difficult
to get right than sex and lighting put together.
References
Corley, L. S., Blankenship,
J. R., Moore, A. J. & Moore, P. J., Developmental
constraints on the mode of reproduction in the
facultatively parthenogenetic cockroach Nauphoeta
cinerea. Evolution and Development1, 90
(1999).
