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'.

Keywords: dioecy; haploid selection; sexual conflict.

Figure 1.

Model of sex chromosome evolution and turnover. (a) Rewiring of the sex determination pathway (dashed line) is a common cause of sex chromosome turnover in animals, and this often involves the change in regulatory position of a gene in the sex determination pathway [48]. In these cases, the chromosome with the new apical sex determining locus has the potential to become a new sex chromosome if recombination suppression develops. The old X chromosome (in red) reverts to being an autosome and the Y (blue) is likely to be lost. (b) Rewiring of the plant two-locus model is somewhat more complicated, as it requires close proximity to dominant female sterility (F^S) male fertility (M) loci on the Y, and recessive female fertility (f^S)and male sterility (m) loci on the X. A turnover of sex chromosomes through the change in regulatory precedence (dashed lines) of the sex determining cascade would require proximity of the new male and female sex determination genes, and this is unlikely, hampering turnover events through this mechanism. Figure by Jacelyn Shu of jacelyndesigns.com.

Figure 2.

Haploid selection and the maintenance of gene activity on the Y chromosome. Proportion of life cycle and proportion of genes expressed in the haploid phase in animals (a) and seed plants (b). Figure by Jacelyn Shu of jacelyndesigns.com.