How Australian mammals contributed to our understanding of sex determination and sex chromosomes

Abstract

Abstract. Marsupials and monotremes can be thought of as independent experiments in mammalian evolution. The discovery of the human male-determining gene, SRY, how it works, how it evolved and defined our sex chromosomes, well illustrates the value of comparing distantly related animals and the folly of relying on humans and mice for an understanding of the most fundamental aspects of mammalian biology. The 25th anniversary of the discovery of SRY seems a good time to review the contributions of Australian mammals to these discoveries.The discovery of the mammalian sex determining gene, SRY, was a milestone in the history of human genetics. SRY opened up investigations into the pathway by which the genital ridge (bipotential gonad) becomes a testis. Studies of Australian mammals were important in the story of the discovery of SRY, not only in refuting the qualifications of the first candidate sex-determining gene, but also in confirming the ubiquity of SRY and raising questions as to how it works. Studies in marsupials also led to understanding of how SRY evolved from a gene on an autosome with functions in the brain and germ cells, and to identifying the ancestors of other genes on the human Y. The discovery that platypus have sex chromosomes homologous, not to the human XY, but to the bird ZW, dated the origin of the therian SRY and the XY chromosomes it defined. This led to important new models of how our sex chromosomes function, how they evolved, and what might befall this gene and the Y chromosome it defines.

Default Sex and Single Gene Sex Determination in Dioecious Plants

“…Whereas if ovaries are removed from young XX embryos, they still develop as females ( Jost, 1970 ), implying that a specific signal was required for male development, but not for female development. For this reason females were considered to be the default sex ( Graves, 2017 ). Later, the gene ‘Sex-determining Region on the Y’ or SRY was discovered ( Sinclair et al., 1990 ) which fitted the requirements of the TDF master regulator that could divert default female development into male development.…”

A convergent mechanism of sex determination in dioecious plants: Distinct sex-determining genes display converged regulation on floral B-class genes

“…(P. tremula) and ZZ/ZW (P. alba) in a simple way (Müller et al, 2020). The plant "one-gene model" was usually thought of as reminiscent of the human sex-determining system that has SRY (Sex-Determining Region Y) gene acting alone as a sex determinant (Sinclair et al, 1990;Graves, 2016). This convergent model thus resembles the bottom-up regulatory model of animal sex determination (Feng et al, 2020).…”

Studies on the molecular underpinnings of sex determination mechanism evolution and molecular sexing tools in turtles

“…Despite the relatively low rates of molecular evolution in turtles, significant differences in evolutionary rates were detected among turtle lineages. For instance, both the nucleotide and amino acid substitution rates of the target genes in Emydidae (Figure 2.2, (Montiel et al 2016b, Rovatsos et al 2016, Ezaz et al 2017, Graves 2017.…”