Relationships between Vertebrate ZW and XY Sex Chromosome Systems

Abstract

The peculiar cytology and unique evolution of sex chromosomes raise many fundamental questions. Why and how sex chromosomes evolved has been debated over a century since H.J. Muller suggested that sex chromosome pairs evolved ultimately from a pair of autosomes. This theory was adapted to explain variations in the snake ZW chromosome pair and later the mammal XY. S. Ohno pointed out similarities between the mammal X and the bird/reptile Z chromosomes forty years ago, but his speculation that they had a common evolutionary origin, or at least evolved from similar regions of the genome, has been undermined by comparative gene mapping, and it is accepted that mammal XY and reptile ZW systems evolved independently from a common ancestor. Here we review evidence for the alternative theory, that ZW<-->XY transitions occurred during evolution, citing examples from fish and amphibians, and probably reptiles. We discuss new work from comparative genomics and cytogenetics that leads to a reconsideration of Ohno's idea and advance a new hypothesis that the mammal XY system may have arisen directly from an ancient reptile ZW system.

Triploid plover female provides support for a role of the W chromosome in avian sex determination

“…In most vertebrate groups, the mechanism of sex determination is not fully conserved [20]. For example, switches between environmental and genetic sex determination (ZW or XY) have occurred frequently during the evolutionary history of reptiles [21][22][23]. Previously, two cases of adult ZZW females were reported in blue-and-yellow macaws Ara ararauna and great reed warblers Acrocephalus arundinaceus [24,25], two other non-galliform species.…”

The role of repetitive DNA in structure and evolution of sex chromosomes in plants

“…In some groups such as birds and lepidopterans, the reverse is true: females are heterogametic (ZW) and males are homogametic (ZZ). Here we will focus on the XY/XX system, but the same processes were observed in ZW/ZZ system as well (Traut and Marec, 1997;Ezaz et al, 2006;Tsuda et al, 2007).…”

Speciation through evolution of sex-linked genes

“…However, there are alternative mechanisms such as environmental sex determination (for example, Sarre et al, 2004) or haplodiploidy, in which one sex is haploid and the other diploid (for example, Hedrick and Parker, 1997). There are also many species with genetic sex determination but with unidentified sex chromosomes (Ezaz et al, 2006;Mank et al, 2006a). The most widely known sex chromosome systems are the XX/XY (females are homogametic and have two copies of the X chromosome whereas males are heterogametic and have one X and one Y) and ZZ/ZW (males have two Z chromosomes and females have one Z and one W) systems.…”