The Homozygosity of Clones of the Self-Fertilizing Hermaphroditic Fish Rivulus marmoratus Poey (Cyprinodontidae, Atheriniformes)

Two clones of the hermaphroditic fish Rivulus marmoratus Poey, identified by their intraclonal histocompatibilities and interclonal histoincompatibility, were kept through successive uniparental generations, each fish in isolation. These hermaphrodites emit internally self-fertilized eggs incubated up to 2 1/2 days intraparentally and thereafter extraparentally. By extraparental incubation at low temperature, primary males can be produced at will from eggs otherwise yielding hermaphrodites. Secondary males originate from hermaphrodites, usually late in life, if the ovarian component of the ovotestis involutes. There are no females. Eggs from an aberrant hermaphrodite chronically emitting un-self-fertilized eggs were cross-fertilized artificially and also naturally by sperm from a primary male. A fin graft from the clone of the male parent was accepted (autograft reaction) by the interclonal hybrid, showing that amphimixis had occurred and that the clone of the graft donor is homozygous for the histocompatibility genes and, by extension, for all or most of the genome. Natural interclonal hybridization was verified by the rejection (allograft reaction) of grafts from putative hybrids by graft hosts belonging to the clone of the hermaphrodite parent. The hybridity of uniparental hybrid F2 fish was confirmed by rejections of grafts from them by graft hosts of either clone. One out of two hybrid F2 intersib grafts was accepted, indicating a difference between the two clones at within one to five histocompatibility loci, or somewhat more if there are linkages between histocompatibility genes or weak histoincompatibilities allowing graft survival. The possible role of males is discussed in view of the evidence of their rarity and for the prevalence of homozygous clones in the wild.