Translocation heterozygosity and the origin of dioecy in Viscum

Summary

Sex-associated and floating translocation complexes are characteristic of dioecious species of Viscum, but are virtually absent in monoecious species. In most of the dioecious species, fixed sex-associated translocation complexes are present, with the male being the heterozygous sex. The sex-associated multivalent is usually ⊙4 (ring-of-four) or ⊙6, rarely ⊙8. In only a few of the dioecious species are sex-associated translocations not apparent. Most of the dioecious species are also polymorphic for floating translocations, producing one or more additional multivalents ranging from ⊙4 to ⊙12. Floating translocations may be more frequent in species which do not have sex-associated translocations. Supernumerary chromosomes are also present in several species. Sex ratios are at unity in most dioecious species, but female-biased ratios as low as 0·52 and male-biased ratios as high as 1·40 may occur in some species. The high correlation between dioecy and translocation heterozygosity suggests that translocations are primarily associated with the origin and establishment of dioecy, and any role they play in the maintenance of biased sex ratios through meiotic drive is probably secondary. A model is presented in which male- and female-determining factors are non-allelic, and sex-associated translocations serve to stabilise dioecy by bringing the sex factors into linkage. Subsequent structural rearrangements within a sex-associated translocation complex may bring the sex factors together in one chromosome pair, releasing floating translocations. The high frequencies of floating translocation heterozygosity in some species indicate that such heterozygosity has adaptive value.