TRANSLOCATION HETEROZYGOSITY, GENETIC HETEROZYGOSITY, AND INBREEDING IN CLARKIA SPECIOSA.

Abstract

Numerous species have been found to have reciprocal translocations as a typical component of their genetic systems (Burnham, 1956). Theoretically, individuals which are heterozygous for reciprocal translocations could have greatly reduced genetic recombination compared to structural homozygotes, and it is commonly agreed this could have important adaptive significance in many natural populations (see Lewis and John, 1963). Upon self fertilization, translocation heterozygotes could produce offspring which are genetically just as heterozygous as the parent. This potential for preserving genetic heterozygosity under conditions of severe inbreeding has been suggested by most workers (e.g. John and Quarishi, 1964) to be the cause for the abundance of floating reciprocal translocations in natural populations of many species. However, there is no theoretical reason that translocation heterozygotes must maintain genetic heterozygosity more efficiently than the corresponding homozygotes under conditions of inbreeding. Further, as discussed by Mooring (1961), numerous alternative factors could be involved in the maintenance of translocation polymorphisms in natural populations. Very few studies to date have provided clear evidence supporting the hypothesis that translocation heterozygosity is correlated in any way with the level of genetic heterozygosity in natural populations. The permanent translocation heterozygote, Oenothera biennis, has been found to be genetically heterozygous for some loci controlling both morphological characters (see Cleland, 1972, for review) and enzymes (Levin et al., 1972). However, studies of intraand interpopulational enzymatic variation have shown little correlation between genic and structural heterozygosity (Levin, 1975). The extraordinarily complicated genetic system in such permanent slocation heterozygotes, including the lack of naturally occurring structural homozygotes, makes difficult a direct assessment of the extent to which translocation heterozygosity affects the maintenance of genetic heterozygosity. In contrast, species having floating translocations permit direct comparisons between translocation heterozygotes and homozygotes. Studies on garden individuals of Chrysanthemum carinatum indicated that plants heterozygous for translocations were larger and more fecund than homozygotes and it was hypothesized that certain coadapted gene complexes were being maintained by the translocations (Rana, 1966). However, without additional information it is not certain whether the translocation heterozygosity is actually maintaining adaptive linkage combinations or whether it is correlated with particular genetic combinations perhaps temporarily. Controlled inbreeding studies on Clarkia williamsonii led Lewis (1969) to conclude that translocation heterozygotes obtained from natural populations of that species did not preserve particular adaptive combinations of genes; progenies derived from self-pollinated chromosomal homozygotes were just as vigorous as progenies from translocation heterozygotes. Experiments with Campanula persicifolia indicate that translocation heterozygosity can mark genetic hetrozygosity (Darlington and LaCour, 1950). In this case translocation heterozygotes were synthesized by hybridizing different naturally occurring chromosomal