Abstract Detail

Insights into the diversity of plant sex chromosomes

Carey, Sarah [1], McDaniel, Stuart [1].

Cryptic evolutionary strata in an ancient UV sex chromosome system.

Sex chromosomes occur in diverse organisms and are complex, dynamic regions of the genome. A defining characteristic of sex chromosomes is suppressed recombination, which is widely believed to cause degeneration, including an accumulation of deleterious mutations and potentially gene loss. Degeneration in turn may drive the compensatory gain of new genes through translocations or the expansion of heterochromatin. In a few well-studied cases, multiple such events form blocks of genes that became sex-linked at the same time, called evolutionary strata. However, other processes involving sexual antagonistic selection also may drive these fusions. Distinguishing between these alternatives is difficult in most old sex chromosome systems, because the homologous genes necessary for evolutionary analysis are degenerate or absent.
Here we sequenced and assembled chromosome-scale genomes for a male and female of the moss Ceratodon purpureus and use these data to examine sex chromosome evolution in mosses. The C. purpureus UV sex chromosomes comprise ~30% of the ~360 Megabase (Mb) genome and harbor ~12% of the gene content. We used synteny-based analyses to identify collinear chromosomes within C. purpureus (i.e., resulting from duplications) and between the hermaphroditic species Physcomitrium patens, which diverged over 200 million years ago. We show the moss genome is comprised of seven remarkably stable ancestral chromosomal elements. One stark exception are the C. purpureus sex chromosomes, which lack any synteny. Instead, we used a phylogenomic approach to reconstruct the history of the modern C. purpureus sex chromosomes. We find the moss sex chromosomes evolved at least 300 million years ago and have expanded from two distinct chromosomal fusions. We found the absence of evolutionary strata from these events is caused by rapid rearrangements within the non-recombining portions of the UV sex chromosomes. Contrary to other ancient sex chromosomes, we found only limited evidence of degeneration of C. purpureus UV sex chromosomes, but rather independent, parallel recruitment of genes in the liverwort Marchantia polymorpha suggests sexual antagonism is likely to have driven these translocations. Together these results highlight the role of the sex chromosomes in driving karyotypic change, and point to a major role for genomic conflict in this change.

1 - University of Florida, Department of Biology, Gainesville, FL, USA

sex chromosomes
Ceratodon purpureus.

Presentation Type: Symposium Presentation
Session: SY5, Insights into the diversity of plant sex chromosomes
Location: Virtual/Virtual
Date: Friday, July 31st, 2020
Time: 11:00 AM
Number: SY5003
Abstract ID:246
Candidate for Awards:None

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