Bdelloid rotifers are a class of microscopic invertebrates that have existed for millions of years apparently without Bdelloid rotifers asexual budding or meiosis. They inhabit a variety of temporary and permanent freshwater habitats globally, and many species are remarkably tolerant of desiccation.
Bdelloids offer an opportunity to better understand the evolution of sex and recombination, but previous work has emphasised desiccation as the cause of several unusual genomic features in this group.
Here, we present high-quality whole-genome sequences of 3 bdelloid species: Rotaria macrura and R. In combination with the published assembly of A. We find that ancestral tetraploidy is conserved among all 4 bdelloid species, but homologous divergence in obligately aquatic Rotaria genomes is unexpectedly low.
This finding is contrary to current models regarding the role of desiccation in shaping bdelloid genomes. In addition, we find that homologous regions in A. "Bdelloid rotifers asexual budding," several features interpreted as genomic evidence for long-term ameiotic evolution are not general to all bdelloid species, even within the same genus.
Finally, we substantiate previous findings of high levels of horizontally transferred nonmetazoan genes in both desiccating and nondesiccating bdelloid species and show that this unusual feature is not shared by other animal phyla, even those with desiccation-tolerant representatives. These comparisons call into question the proposed role of desiccation in mediating horizontal genetic transfer. Bdelloid rotifers are microscopic animals that live in freshwater habitats throughout the world.
Two life history characteristics distinguish these common invertebrates as extraordinary. First, they have existed for millions of years apparently without sex: Second, most bdelloid species are highly resilient to desiccation and can survive without water Bdelloid rotifers asexual budding extended periods of time a process known as anhydrobiosis.
These 2 attributes have been predicted to leave signatures in bdelloid genomes. We find that many proposed genomic consequences of asexuality and desiccation tolerance do not hold true for all species. The genomes of bdelloids may be more similar to those of other animals than previously thought, though a remarkable exception is the high proportion of genes acquired horizontally from nonmetazoan taxa.
Our findings necessitate a reevaluation of the effects of asexuality and desiccation on genome evolution in bdelloid rotifers.
The bdelloid rotifers are a class of microscopic Bdelloid rotifers asexual budding found in freshwater habitats worldwide. Two life history characteristics make these soft-bodied filter feeders unusual among animals. First, bdelloids famously lack males [ 1 ] or cytological evidence of meiosis [ 23 ] and are only known to reproduce via Bdelloid rotifers asexual budding parthenogenesis.
They are therefore one of the best-substantiated examples of a eukaryotic taxon that has evolved apparently without sex or meiosis for tens of millions of years [ 1245 ]. Their persistence has implications for theories of the evolution of sex and recombination, a fundamental puzzle in biology [ 12 — 15 ].
A second key feature is that most bdelloid species are remarkably tolerant of desiccation and can survive the loss of almost all cellular water any stage in their life cycle, including as adults [ 1617 ].
Individuals can remain in this condition for long periods, usually days or weeks but occasionally several years [ 1920 ]. The return of water restores metabolism and reproduction, with little evidence of negative fitness consequences for survivors [ 21 ].
Species that live in limnoterrestrial habitats such as puddles, leaf litter, and moss are subject to rapid and repeated cycles of drying.
The ability to survive desiccation has been proposed to play a key role in bdelloid evolution [ 522 ]. Bdelloid rotifers asexual budding marker-based analyses of bdelloid genomes recovered highly divergent gene copies that were interpreted as nonrecombining descendants of ancient former alleles [ 4 ].
Along with the low copy number of vertically inherited transposable elements TEs [ 23 ], this result was considered positive genetic evidence of long-term asexual evolution. However, subsequent investigations of larger genomic regions revealed evidence of tetraploidy, probably arising from an ancient hybridisation or genome duplication event affecting diploid ancestors prior to the diversification of bdelloid families [ 52425 ].
Another extraordinary feature was that a remarkably high proportion of bdelloid genes show similarity to nonmetazoan orthologs, mostly from bacteria but also fungi and plants, suggesting a rate of horizontal gene transfer HGT into bdelloid genomes at least an order of magnitude greater than that observed in other eukaryotes [ 26 ]. Many genes originating by HGT from nonmetazoans are expressed and functional [ Bdelloid rotifers asexual budding27 ].