Young sex chromosomes in plants and animals

Abstract

A major reason for studying plant sex chromosomes is that they may often be 'young' systems. There is considerable evidence for the independent evolution of separate sexes within plant families or genera, in some cases showing that the maximum possible time during which their sex-determining genes have existed must be much shorter than those of several animal taxa. Consequently, their sex-linked regions could either have evolved soon after genetic sex determination arose or considerably later. Plants, therefore, include species with both young and old systems. I review several questions about the evolution of sex-determining systems and sex chromosomes that require studies of young systems, including: the kinds of mutations involved in the transition to unisexual reproduction from hermaphroditism or monoecy (a form of functional hermaphroditism); the times when they arose; and the extent to which the properties of sex-linked regions of genomes reflect responses to new selective situations created by the presence of a sex-determining locus. I also evaluate which questions are best studied in plants, vs other suitable candidate organisms. Studies of young plant systems can help understand general evolutionary processes that are shared with the sex chromosomes of other organisms.

Multiple evidences suggest sox2 as the main driver of a young and complex sex determining ZW/ZZ system in turbot (Scophthalmus maximus)

“…Further, this strategy also allowed us to detect minor differences that could be related to the causative mutation determining sex in turbot. As previously suggested by Taboada et al (2014), we confirm that the major SD region of turbot is a young one (Charlesworth, 2019), since no consistent genetic differences between Z and W chromosomes more than in a single nucleotide (SNP.3) could be detected. In this regard, turbot's would be among the youngest sex chromosome pairs found in fish at a stage similar to that of the pufferfish (Fugu rubripes; Kamiya et al, 2012), where the only difference between X and Y chromosomes was a single differential SNP at the promoter of the amh receptor.…”

“…We got a comprehensive picture of the complex mechanism underlying SD in this species and framed this information within teleost SD evolution. In particular, our work represents a contribution to the knowledge of the origin and evolution of "young sex chromosome pairs" (Charlesworth, 2019), still very unknown unlike the most studied mammalian, avian and Drosophila systems, which represent "old sex chromosomes" with specific genetic features: size heteromorphism, specialized gene content, reduced recombination and degeneration (Abbott et al, 2017).…”

Widespread Recombination Suppression Facilitates Plant Sex Chromosome Evolution

“…Indeed, low rates of recombination are predicted to increase the likelihood of the maintenance of sexually antagonistic polymorphisms (18, 22, 40, 42). Thus, regions of the genome with low rates of recombination may be generally predisposed to evolve sex-linked regions (40). Indeed, evidence for a role for ancestrally low rates of recombination in the evolution of sex chromosomes has started to emerge in plants, as reported in papaya, Carica papaya , (43) and kiwifruit, Actinidia chinensis , (45).…”

“…Low rates of recombination may allow for the invasion of recombination modifiers completely linking haploid-expressed (pollen) genes to the Y (50, 54). Therefore, there are theoretical reasons to expect that pre-existing low-recombination plays a crucial role in sex-chromosome formation (40), and in both kiwifruit and papaya, sex-determining regions originated in centromeric regions that were likely ancestrally recombination-suppressed (43, 44). In both of these systems, however, the sex chromosomes are small and homomorphic or micro-heteromorphic.…”

Sex‐chrom, a database on plant sex chromosomes

“…These regions show signs of chromosomal rearrangements, inversions and translocations typical for the nonrecombining region of the Y chromosome (Ming et al, 2007). Papaya constitutes an important example of the origin of the SDR in the chromosomal part already showing reduced recombination (Charlesworth, 2019).…”