Stabilised heterozygosis of supergenes mediating certain Y-linked colour patterns in populations of lebistes reticulatus

Abstract

1. Using methods of artificial sex reversal with mammalian hormones, and employing as an X-chromosome marker a dominant X-linked mutation for body colouration relatively newly arisen in aquarium stock of Lebistes (frequently called “Half-Black”) which is not sex-limited in expression and becomes phenotypically evident in young only a few days old, it has been possible to prepare at will considerable numbers of XY females for use in investigating the viability and fertility of YY males and of females of constitution both XY and YY. 2. Using such material, it has been demonstrated that YY males homozygous at the “absolutely” Y-linked locus Maculatus are non-viable, suggesting the presence of a recessive lethal factor closely linked to the Maculatus locus. This confirms earlier more limited findings of Winge. 3. Winge demonstrated that males heterozygous for Maculatus and for a second “absolutely” Y-linked colour pattern supergene, Pauper, were, on the other hand, fully viable and fertile, suggesting either that there is no corresponding recessive lethal closely linked to Pauper, or that, if such a lethal is present, it is non-allelic to that linked to the Maculatus locus. This latter conclusion would be most interesting, but these alternative possibilities cannot be distinguished from Winge's data. 4. YY males heterozygous for Maculatus and for Armatus, a third “absolutely” Y-linked pattern originally described by Winge, and also heterozygous for Pauper and Armatus have been prepared. Both proved fully viable and fertile. Males homozygous for Pauper and Armatus, however, proved lethal, like those homozygous for Maculatus. This seems to offer good evidence that a series of non-allelic, recessive lethals may be linked to Maculatus, Armatus and Pauper. 5. In back-crosses to females of constitution X0X0 (respecting sex-linked colour patterns) males heterozygous for both Maculatus and Armatus and Pauper and Armatus produced, as expected, approximately equal numbers of male progeny of constitution XOYM5 and XOYAr and YOYPa and XOYAr respectively, indicating that cross-over between Y chromosomes had not occurred. 6. A single example of a fully viable YY male homozygous for Maculatus is described. In this individual the lethal may have been modified in, or eliminated from, one of the Y chromosomes, so that it was, in effect, heterozygous for the lethal. In crosses with X0X0 females it proved fully fertile, giving rise to numerous male progeny only, all of the Maculatus pattern. This male may well have illustrated a situation generally analogous to that described by Yamamoto in Oryzias, and interpreted by him as a consequence of cross-over between the colour locus and an inert chromosome region adjacent to it. 7. Tests were made of the viability and fertility of XY males and XX females homozygous for certain colour pattern supergenes which show measurable cross-over values between Y and X. Both the supergene Coccineus- Vitellinus and Melano-Dorsal, segregated from wild populations in Trinidad, were associated with full viability and fertility when homozygous in both males and females, suggesting the absence here of recessive lethals. 8. It seems evident that heterozygosity in males with respect to the three “absolutely” Y-linked colour factors so far investigated can be importantly stabilised in populations containing them through the action of a series of closely linked recessive genes which are lethal when homozygous, but which appear to be non-allelic among the three pattern supergenes. The possibility that this may be a rather general mechanism stabilising male heterozygosity in wild populations of Lebistes is being investigated, using a series of pattern supergenes from wild Trinidad populations that are similarly “absolutely” Y-linked. 9. The question of whether the non-allelism of the lethal factors linked to Maculatus, Armatus, and Pauper can be referred to single loci or reflect polygenic phenomena within the respective Y chromosomes is interesting and significant. For the present, it will be conservative to refer to the Maculatus, Pauper, and Armatus-bearing Y chromosomes as a whole as nonallelic. What may be especially interesting in the present context is the fact that such non-allelic chromosomes can (and regularly do) coexist, often in quite balanced and stable proportions, within freely interbreeding local natural populations of Lebistes. © 1970, The Genetical Society of Great Britain. All rights reserved.