Sex, does everyone need it? Apparently not, the bdelliod rotifer Adineta vaga doesn’t

by Tayara Favarao and Stuart Brown

The loss of sexual reproduction in metazoans (animals) is widely believed to be an evolutionary dead end and would lead to the extinction of most metazoans. There is of course an exception, the bdelloid rotifers Adineta vaga (see figure 1).

Figure 1: The bdelloid rotifers Adineta vaga
Figure 1: The bdelloid rotifer Adineta vaga

These strange, weird and wonderful creatures seriously challenge this view that sexual reproduction is necessary for long-term evolutionary success. Rotifers of the Bdelloidea Class are a common group of asexual freshwater invertebrates that have persisted for millions of years. Never have we yet observed the existence of male sex organs or even meiosis in these bizarre animals. Their only recorded method of reproduction is via mitotically produced eggs with no reduction in chromosome number or any form of chromosome pairing. However, it is also thought that the bdelliod rotifers could have sex on exceedingly rare occasions and in a cryptic way yet to be defined.

Recent research by Flot et al (2013) looked at the genetic landscape of the bdelloid rotifer Adineta vaga, and they interpret their findings as evidence for bdelloids having evolved ameiotically. Shotgun sequencing on 454 and Illumina platforms were used for the genetic analyses. The chromosomes of the rotifers have a strange structure that means that it is impossible for the genome to split into haploid sets required for meiotic division. To combat their lack of recombination and keep the amount of deleterious mutations low, A. vaga have frequent events of gene conversion, which is when one allele of the same gene replaces another allele, these events promotes repair of mutations that arise (see figure 2). The rotifers also have expanded genomic regions that prevent transposons, jumping genes that often arise by duplication or cut themselves from one part of the genome and paste themselves into another part. Rotifer transposons comprise only 3% of the rotifers 244 Mb (Megabase) genome size, in humans transposons make up 44% of our 3300 Mb genome size. Rotifers have over twice the amount of genes (~49300) than humans (~21000) where over 8% of the rotifer genes are from non-metazoan origin, harking at a prokaryotic-like lifestyle involving horizontal gene transfer.

Figure 2: In sexual organisms, meiotic recombination can generate offspring with fewer or more deleterious mutations (hence increasing or decreasing fitness) than the previous generation. The same outcome is expected in ameiotic organisms that experience gene conversion: a deleterious allele may be overwritten by a beneficial or neutral one, resulting in an increase in fitness, or may overwrite it, resulting in decreased fitness.
Figure 2: In sexual organisms, meiotic recombination can generate offspring with fewer or more deleterious mutations (hence increasing or decreasing fitness) than the previous generation. The same outcome is expected in ameiotic organisms that experience gene conversion: a deleterious allele may be overwritten by a beneficial or neutral one, resulting in an increase in fitness, or may overwrite it, resulting in decreased fitness.

Flot and his colleagues found in the A. vaga   genome, evidence of genomic palindrome, which is when the DNA sequence has a complement sequence that when reversed shows the original sequence of nucleotide-by-nucleotide, such as: ACCTAGGTIS (original sequence),SITGGATCCA (complement sequence) and ACCTAGGTIS (complement sequence reverted). After analysing palindromic regions they found that there was colinearlity and divergence signatures of allelic regions and that there were no other allelic duplicates in the assembly. Suggesting that they arose by inter-allelic rearrangements rather than local duplication. They also found three direct repeats that present the signatures of allelic blocks found on the same chromosome, which cannot segregate during meiosis. A. vaga genome structure suggests that they have a mitotic lineage.

The frequent cycles of desiccation and rehydration that A. vaga undergo causes breaks in their DNA double-strand, facilitating the integration of the horizontally transferred genetic material and promotes gene conversion when repaired. These processes are believed to replace the role of sex in gene homogeneity and diversity.

This paper showed a new insight of how some species might be able to maintain health genes and viable lineages without the use of sexual reproduction. Hence providing a new possibilities perspective on asexual reproduction.

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