Translocation heterozygosity and sex ratio in Viscum fischeri

Abstract

SUMMARYMale plants of V. fischeri have 2n = 23 and constantly produce seven bivalents and a multivalent chain of nine chromosomes at meiosis. Regular assortment results in transmission of 11-and 12-chromosome genomes via the pollen. Female plants have the chromosome number 2n = 22 and are homozygous for the 11-chromosome genome. The multivalent chain in the males is a consequence of reciprocal translocations, one of which was Robertsonian and one of which involved the chromosome carrying the sex determination factors. There is a constant female-predominant sex ratio of approximately 1:2 in V. fischeri, possibly maintained by gamete selection; the genes involved may have been linked with the sex determination mechanism through the translocation system.

The genetic contribution to sex determination and number of sex chromosomes vary among populations of common frogs (Rana temporaria)

“…Co-segregation of multiple sex chromosomes has notably been documented in some populations of Rana tagoi, where male heteromorphy for C-banding patterns suggests that both chromosome pairs 8 and 9 co-segregate as sex chromosomes (Ryuzaki et al, 1999). In some cases multiple translocations are involved, resulting in a multivalent chain of chromosomes during male meiosis (see, for example, Barlow and Wiens, 1976;Syren and Luykx, 1977;Grützner et al, 2004;Gazoni et al, 2012).…”

Steps in the evolution of heteromorphic sex chromosomes

“…Neo-sex chromosome systems are known in many plants and animals, including XY 1 Y 2 in the plant Rumex acetosa (Rejó n et al, 1994), and multiple sex chromosomes due to translocations in African misteltoes (Barlow and Wiens, 1976), termites (Syren and Luykx, 1977), African pygmy mice (Veyrunes et al, 2004) and monotremes (Rens et al, 2004). As we shall describe below, the sex chromosomes of eutherian mammals represent an ancient case of neo-sex chromosomes, established after the divergence of eutherians and marsupials (Waters et al, 2001).…”

The Birds and the Bees and the Flowers and the Trees: Lessons from Genetic Mapping of Sex Determination in Plants and Animals

“…XY 1 Y 2 neosex chromosome systems have arisen in Humulus japonicus (Kihara and Hirayoshi 1932;Kim et al 2008), and Silene diclinis (Howell et al 2009), and twice independently in Rumex (Smith 1963(Smith , 1969Navajas-Perez et al 2005), and Viscum fischeri has a multiple Y system (Barlow and Wiens 1976). These situations probably evolved by X-autosome translocations (Figure 1; changing the ancestrally autosomal element into a chromosome that segregates from the new arm of the X, denoted by Y 2 , while Y 1 denotes the ancestral Y), although fission of the Y cannot yet be ruled out in Rumex; genetic maps using genic markers might be able to establish whether the Y chromosomes still carry functional genes (as in Silene; see for instance Marais et al 2008), in which case their gene content could be compared and this possibility tested.…”