The Diversity of Plant Sex Chromosomes Highlighted through Advances in Genome Sequencing

Abstract

For centuries, scientists have been intrigued by the origin of dioecy in plants, characterizing sex-specific development, uncovering cytological differences between the sexes, and developing theoretical models. Through the invention and continued improvements in genomic technologies, we have truly begun to unlock the genetic basis of dioecy in many species. Here we broadly review the advances in research on dioecy and sex chromosomes. We start by first discussing the early works that built the foundation for current studies and the advances in genome sequencing that have facilitated more-recent findings. We next discuss the analyses of sex chromosomes and sex-determination genes uncovered by genome sequencing. We synthesize these results to find some patterns are emerging, such as the role of duplications, the involvement of hormones in sex-determination, and support for the two-locus model for the origin of dioecy. Though across systems, there are also many novel insights into how sex chromosomes evolve, including different sex-determining genes and routes to suppressed recombination. We propose the future of research in plant sex chromosomes should involve interdisciplinary approaches, combining cutting-edge technologies with the classics to unravel the patterns that can be found across the hundreds of independent origins.

Keywords: bryophytes; dioecy; seed plants; sex determination; two-gene model; whole-genome sequencing.

Figure 1

Recently discovered angiosperm sex-determination genes. Sex-determining genes recently identified that are involved with carpel development (a) include SyGl, SOFF, and LOG-like. When these genes are expressed (+) in males, it suppresses the function or development of the carpel. However, the lack of expression (-) in females allows for functional carpel development. In grapes, it is not yet known whether VviYABBY3 or VviAPT3 is the female-sterility gene. Several genes have also been identified for promoting stamen function (b). FrBy and TDF1 both promote tapetum development (in blue) and VviINP1 promotes pollen development (in red). It is unknown yet whether CYP703 or GPAT3 is the male-determining gene in date palm, but both are involved in pollen and/or anther development. In persimmon and poplar, a single gene is involved in sex determination (OGI and ARR17, respectively). When MeGI is expressed, flowers develop functional carpels, but not stamens. However, when the Y-linked OGI is expressed, it represses MeGI, resulting in functional stamens. Similarly, in poplars, ARR17 expression results in carpel production, but the lack of expression results in functional stamens.