Debunking myths on genetics and DNA

Tuesday, October 1, 2013

Ms. Stick Insect

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You're looking at a stick insect, a critter I was quite used to growing up as my dad, an evolutionary biologist, used to grow them at home. I know, most households have cats, dogs, guinea pigs and rabbits; ours had cats, dogs, toads, fruit flies, and stick insects. :-)

Children have a tendency to personify everything, animals in particular, so imagine my shock when my dad told me that stick insects are all... ladies. Yup. It's Ms. Stick Insect. And the reason why I mention this is that today I'd like to talk about sex. Ha! You didn't see that coming, did you?

How does an all-female population manage to reproduce? Embryos develop from eggs using parthenogenesis, without the need to be fertilized. This doesn't mean that the offsprings will be identical to the parent. "Reshuffling" of genes is still ensured by meiosis.

In organisms that reproduce sexually, meiosis produces gametes, cells that carry half of the chromosomes and therefore, once fused with the opposite sex gamete, it will produce a cell with the full number of chromosomes. In organisms that reproduce sexually, meiosis produces gametes, cells that carry half of the chromosomes and therefore, once fused with the opposite sex gamete, it will produce a cell with the full number of chromosomes. In diploid organisms (organisms that have two copies of each chromosome), meiosis takes place in the following steps: (i) DNA replication, which creates two exact copies of each chromosome; (ii) pairing of the chromosome homologs, one maternal and one paternal; (iii) the homologs' cross-over creating a unique mix of maternal and paternal DNA; (iii) another round of cell division creates four cells, each with one set of chromosomes.

In parthenogenesis meiosis, step (i) is skipped. In order to restore the two copies of chromosomes, in some perhenogenetic animals, the cell division in step (iv) creates two cells instead of four, each with two copies of chromosomes. However, stick insects employ a different strategy: step (iv) still creates four cells, of which only one has the cytoplasm. This cell then fuses with one of the other three effectively creating and egg with two copies of chromosomes, perfectly equivalent to a fertilized egg.

Not all stick insects reproduce through parthenogenesis. Some populations do have males and mate, though usually only about 10% of offsprings come from sexual reproduction. Morgan-Richards et al. [1] compared several populations of New Zealand stick insects (C. hookeri), and found that while mated females produced male and female offsprings in equal numbers, virgin females that reproduced via parthenogenesis produced mostly females. That's right, I said "mostly".

"A single male hatched from an egg laid by a captive virgin mother. [...] This male may have arisen by the loss of an X chromosome during cell division (non-disjunction), a mechanism recorded for other stick insect species with the same XO⁄XX sex-determination mechanism seen in C. hookeri [1]."

So even in completely parthenogenetic populations, in principle sexual reproduction is not completely lost as the reshuffling provided by meiosis can, occasionally, originate a male offspring. Furthermore, the authors confirmed a geographical distribution of the parthenogenetic population of stick insects compared to the sexual ones: all female populations in New Zealand tend to be more common farther away from the equator and at higher altitudes, implying the adaptive advantage of parthenogens in certain environments but not in others.

The fact that parthenogens would have an adaptive advantage intrigued me, so I dug a bit further and found out about a concept called the two-fold cost of sex. In a sexual population, only one of the two sexes bares offsprings, while in a one-sex population all individuals bare offsprings, hence significantly increasing its growth rate. This seems to indicate that asexual populations have a higher Darwinian fitness. So, how did we end up with so many sexual species given especially that we all originated from asexual ancestors? How can sex be evolutionary successful when the odds seem to be against it?

I'll save that discussion for the next post. :-)

[1] MARY MORGAN-RICHARDS,, STEVE A. TREWICK,, & IAN A. N. STRINGER (2010). Geographic parthenogenesis and the common tea-tree stick insect of New Zealand Molecular Ecology DOI: 10.1111/j.1365-294X.2010.04542.x

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