INDEPENDENT EVOLUTION OF A POLYGENIC SYSTEM IN ISOLATED POPULATIONS OF THE FUNGUS SCHIZOPHYLLUM COMMUNE.

Abstract

One of the aims of studying the heritable variation in natural populations is to give an insight into the forces of selection to which the populations are subject. Thus it is possible to deduce from the genetic control of a character the nature of the selective forces to which the character has been subjected in the past. It can be argued, for instance (Mather, 1966), that under stabilizing selection, only little dominance evolves and this tends to be ambidirectional; i.e. at some loci the increasing alleles are dominant while at other loci the reverse is true. When, on the other hand, selection is directional, strong unidirectional dominance evolves as well as nonallelic interactions. In a series of biometrical studies in the Hymenomycetes, we have investigated the variation existing within natural populations. We have assessed the heritable variation revealed among the dikaryotic progenies of single wild isolates of Schizophyllum commune (Simchen, 1966), as well as the variation in a population of dikaryons of Collybia velutipes which share in a common gene pool (Simchen, 1965). The experiments to be reported in the present paper were performed in order to explore the relationship between the systems controlling in S. commune in strains which were geographically isolated from each other to varying degrees. It was expected that if the polygenic system has evolved independently in two populations, individuals which contain nuclei from different origins in dikaryotic association will display genetic interaction in a manner which cannot be predicted, since alien genetic elements have not been selected to cooperate with each other. THE CROSSES In S. commune, as in other related fungi, two distinct and independent phases exist in the life cycle: the monokaryon, which is virtually a haploid organism, each cell containing one and the same haploid nucleus; and the dikaryon which behaves very similar to a diploid organism, in spite of each cell containing a pair of haploid nuclei and not one diploid nucleus. The character under investigation in the experiment reported here was ten days' (in tubes) of the dikaryotic mycelia, referred to throughout as growth rate. The experimental and statistical methods employed are essentially the same as the on,es described in detail by Simchen and Jinks (1964). The behavior of dikaryotic rate in multiple-cross programs, when carried out among the progeny of any single isolate, is compatible with a genetic model assuming additivity and dominance only. In the present study the isolates which gave the best agreement with the genetic model were crossed. These proved to be isolates 1, 2, and 5 (Simchen, 1966: Figure 1 and Table 5). The three wild dikaryons are related to one another to varying degrees. Isolate 5 was collected in Massachusetts, USA, and the two other isolates were collected from locations in England which were 50 km apart. While there can be no gene flow between the American and the English populations, it is possible that there is some gene exchange between the two English populations. In addition to their different degrees of isolation, these three dikaryons provide us with three degrees of heterozygosity; isolates 5 and 1 having the highest and the lowest levels of heterozygosity respectively.