Are plant and animal sex chromosomes really all that different?

Abstract

Sex chromosomes in plants have often been contrasted with those in animals with the goal of identifying key differences that can be used to elucidate fundamental evolutionary properties. For example, the often homomorphic sex chromosomes in plants have been compared to the highly divergent systems in some animal model systems, such as birds, Drosophila and therian mammals, with many hypotheses offered to explain the apparent dissimilarities, including the younger age of plant sex chromosomes, the lesser prevalence of sexual dimorphism, or the greater extent of haploid selection. Furthermore, many plant sex chromosomes lack complete sex chromosome dosage compensation observed in some animals, including therian mammals, Drosophila, some poeciliids, and Anolis , and plant dosage compensation, where it exists, appears to be incomplete. Even the canonical theoretical models of sex chromosome formation differ somewhat between plants and animals. However, the highly divergent sex chromosomes observed in some animal groups are actually the exception, not the norm, and many animal clades are far more similar to plants in their sex chromosome patterns. This begs the question of how different are plant and animal sex chromosomes, and which of the many unique properties of plants would be expected to affect sex chromosome evolution differently than animals? In fact, plant and animal sex chromosomes exhibit more similarities than differences, and it is not at all clear that they differ in terms of sexual conflict, dosage compensation, or even degree of divergence. Overall, the largest difference between these two groups is the greater potential for haploid selection in plants compared to animals. This may act to accelerate the expansion of the non-recombining region at the same time that it maintains gene function within it. This article is part of the theme issue ‘Sex determination and sex chromosome evolution in land plants’.

Chromosome-scale assembly with a phased sex-determining region resolves features of early Z and W chromosome differentiation in a wild octoploid strawberry

“…More data are especially needed from gene expression, as sex-biased expression is often used as a proxy for sexual conflict ( Mank 2017 ). It is important to note that while sexual conflict can play an important role in sex chromosome evolution in animals, it seems limited in plants; in contrast, plants display a higher potential for haploid selection ( Mank 2022 ). Due to the variability of the potential drivers of sex chromosomes in animals, for example mutation load in Ranid frogs ( Jeffries et al 2018 ) vs sexual antagonistic selection in cichlid fish ( Roberts et al 2009 ), it is important to compare animal and plant systems that possess a similar trajectory of sex chromosome evolution.…”

Does polyploidy inhibit sex chromosome evolution in angiosperms?

“…However, SLR regions of dioecious plants may be less subject to accumulation of deleterious mutations than in animals, because haploid gametophytes express up to 65% of genes (Joseph and Kirkpatrick, 2004;Gorelick, 2005;Cronk, 2022; 10.3389/fpls.2022.976765 Mank, 2022). Hence, haploid selection is probably much stronger in plants than in animals (Otto et al, 2015), and could prevent sex chromosome degeneration.…”