By Donna M. Recktenwalt
Population geneticists are well aware that there is great variation in the amount of inbreeding that occurs in nature and the resulting inbreeding depression that often occurs in captivity. However, the mechanics that cause this effect are not obviously clear.
For example, in Rivulus marmoratus most individuals are completely homozygous (alike), and natural populations (with a few exceptions) are simply arrays of homozygous clones. Yet this species inhabits one of the most challenging environments in nature, mangrove swamps that endure great fluctuations of salinity, temperature, etc.
Also, peripheral populations of Xiphophorus maculatus show little inbreeding depression in the laboratory, while those from larger and more centrally located populations show strong depression effects in just a few generations of inbreeding.
There seem to be two important active variables at work here: effective population size and the extent of inbreeding in nature. Large populations tend to accumulate deleterious recessive genes; when these populations are inbred in the laboratory, these recessives (which would persist indefinitely-and probably harmlessly-in the heterozygous form) become homozygous and result in inbreeding depression. The mating systems of some populations favor inbreeding (the self-fertilization of R. marmoratus is its ultimate form). These populations gradually eliminate the deleterious recessives without much inbreeding depression in any given generation.
This leads to two questions: how big are the wild populations? And do they have mating systems that favor inbreeding? If only a few males in a large population do most of the breeding, the effective population sizes may be smaller than expected.
There is also such a thing as outbreeding depression, most often expressed as F1 sterility. This might be encountered when crossing lines derived from populations which (unknown to us) are genetically incompatible. Chromosomal changes (“mutations”) often lower the fertility of heterozygous carriers, since they can result in unbalanced chromosome sets. Other variants, such as Robertsonian translocations (another form of chromosomal transfer) also may occur. The results of such mutations usually die early. If two populations happen to be fixed for several chromosomal differences, any hybrids between them tend to be sterile. Sometimes, by normal genetic drift, a mutation can become fixed in one population but not another; hybrids between two such populations often are viable, but have reduced fertility.
Many of the West African killifish, particularly from the Fundulopanchax and Aphyosemion groups, are of interest to evolutionary geneticists because they consist of populations with different multiple Robertsonian translocations. As Joergen Scheel showed (see Rivulins of the Old World, the first edition) hybrids between these fish are often viable, but sterile. Basically, these groups of fish are arrays of chromosomally differentiated “sibling” species.
Thus the idea of “introducing new blood” by crossing wild-caught individuals into established stocks is risky. In the absence of sufficient genetic information or precise locality data, it should not be attempted.
— G.C.K.A. Newsletter, November 1998