Evolution 2019: On the Origin of Anolis Sex Chromosomes

The origin and maintenance of reproductive isolation between species is a central question to evolutionary biologists. Divergent sex chromosomes can play an important role in this process, and are generally assumed to have outsized importance in the establishment of reproductive barriers. Studying the origin and evolution of sex chromosomes – and their respective fusions and fissions – may therefore provide key insights into their role in these processes.

Anole are known to vary in sex chromosome size and content, although all anoles are male heterogametic. In a poster at Evolution, de Mello et al. investigate the neo-sex chromosomes of Anolis distichus, one of the “model anoles” of speciation research. Starting from a newly assembled genome, these researchers used differences in coverage, k-mer comparisons, and synteny mapping to the Anolis carolinensis genome, to identify the sex-linked genomic regions of A. distichus.

From these results, de Mello et al. were able to identify deep conservation of the X chromosome between A. distichus and A. carolinensis – implying an ancient origin of a shared anole X chromosome. They also identified explicitly Y-linked scaffolds for the first time in any Anolis species, which will prove useful for future work on the evolution of these sex chromosomes. However, perhaps most excitingly, de Mello et al. identified a chromosome fusion of the Anolis carolinensis microchromosomes 11 and 12 to the A. distichus X chromosome. In other words, the A. distichus  X chromosome has expanded through the fusion of these two microchromosomes.

de Mello et al.’s  result that the  A. distichus sex chromosomes are simultaneously ancient and newly expanding provides a fascinating look at the dynamic lives of these sex chromosomes. Future investigations into the evolution of Anolis sex chromosomes will surely prove fruitful to understanding their role in the diversification of the Anolis lineages.