Population Genomics of Two Closely Related Anhydrobiotic Midges Reveals Differences in Adaptation to Extreme Desiccation

The sleeping chironomid Polypedilum vanderplanki is capable of anhydrobiosis, a striking example of adaptation to extreme desiccation. Tolerance to complete desiccation in this species is associated with emergence of multiple paralogs of protective genes. One of the gene families highly expressed under anhydrobiosis and involved in this process is protein-L-isoaspartate (D-aspartate) O-methyltransferases (PIMTs). Recently, another closely related midge was discovered, Polypedilum pembai, which is able not only to tolerate desiccation but also to survive multiple desiccation-rehydration cycles. To investigate the evolution of anhydrobiosis in these species, we sequenced and assembled the genome of P. pembai and compared it with P. vanderplanki and also performed a population genomics analysis of several populations of P. vanderplanki and one population of P. pembai. We observe positive selection and radical changes in the genetic architecture of the PIMT locus between the two species, including its amplification in the P. pembai lineage. In particular, PIMT-4, the most highly expressed of these PIMTs, is present in six copies in the P. pembai; these copies differ in expression profiles, suggesting possible sub- or neofunctionalization. The nucleotide diversity of the genomic region carrying these new genes is decreased in P. pembai, but not in the orthologous region carrying the ancestral gene in P. vanderplanki, providing evidence for a selective sweep associated with postduplication adaptation in the former. Overall, our results suggest an extensive relatively recent and likely ongoing adaptation of the mechanisms of anhydrobiosis.