Genomic evidence for ameiotic evolution Adineta vaga
Democratization of genome sequencing has given power to researchers asking all kinds of interesting biological question, and not limiting genomics to what the bureaucrats at cancer agency think would be good for their cure cancer by 2015 propaganda. Here is a fascinating paper from an international collaboration led by a French group.
[Genomic evidence for ameiotic evolution in the bdelloid rotifer Adineta vaga] (http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12326.html)
Loss of sexual reproduction is considered an evolutionary dead end for metazoans, but bdelloid rotifers challenge this view as they appear to have persisted asexually for millions of years1. Neither male sex organs nor meiosis have ever been observed in these microscopic animals: oocytes are formed through mitotic divisions, with no reduction of chromosome number and no indication of chromosome pairing2. However, current evidence does not exclude that they may engage in sex on rare, cryptic occasions. Here we report the genome of a bdelloid rotifer, Adineta vaga (Davis, 1873)3, and show that its structure is incompatible with conventional meiosis. At gene scale, the genome of A. vaga is tetraploid and comprises both anciently duplicated segments and less divergent allelic regions. However, in contrast to sexual species, the allelic regions are rearranged and sometimes even found on the same chromosome.
Such structure does not allow meiotic pairing; instead, we find abundant evidence of gene conversion, which may limit the accumulation of deleterious mutations in the absence of meiosis. Gene families involved in resistance to oxidation, carbohydrate metabolism and defence against transposons are significantly expanded, which may explain why transposable elements cover only 3% of the assembled sequence. Furthermore, 8% of the genes are likely to be of non-metazoan origin and were probably acquired horizontally. This apparent convergence between bdelloids and prokaryotes sheds new light on the evolutionary significance of sex.
What do we like?
1. Use of MIRA+SSPACE assembler - that is non-mainstream for genome papers.
2. Genome assembly alone led to interesting findings. Fig.3 - “A genome structure incompatible with conventional meiosis” covers their main point.
3. Very low amount (3%) of transposons in the genome.
4. The term ‘ohnologous’ getting more accepted.
5. HOX:
We found that A. vaga includes many more homeobox gene families with four copies than are seen in other animals, including vertebrates that have undergone two rounds of genome
duplication and even the zebrafish that has undergone an additional round (Supplementary
Fig. 21). This inordinate maintenance of four gene copies likely points to constraints on gene
balance during embryogenesis counterselecting the independent loss of gene copies is
counterselected.
The paper is full of interesting nuggets and is worth taking a look. We will add to the commentary as we continue to read through it.
