Alleles In Natural Populations Research Articles

The Effect of Migration on the Maintenance of a Lethal Polymorphism in the House Mouse

Most wild populations of house mice are polymorphic for alleles at the Brachury, T, locus. These alleles are either recessive lethals or viable male steriles and are maintained by an abnormal segregation (transmission) ratio in heterozygous males. The observed frequencies of t alleles in natural populations are considerably less than those expected from deterministic models. This discrepancy has been attributed to the operation of random genetic drift in the small, relatively isolated breeding units (demes) of which a mouse population is comprised. In this investigation, a Monte Carlo simulation of a geographical population has been employed to examine the range of breeding-unit sizes and interdemic migration rates for which the random drift would serve as a likely explanation for the low observed frequencies of the t alleles. In the runs made, lethal t alleles with a transmission ratio of 0.95 were examined. The results of these runs indicate that random drift would only have an important effect on t allele frequencies in populations composed of totally isolated breeding units with a genetically effective size of less than 12 individuals. With 1% migration between demes, the effective breeding-unit size would have to be less than eight. With 3% migration, random drift would only have an important effect on t allele frequencies if the breeding units had a genetically effective size of less than four. Current estimates of these parameters obtained from biochemical polymorphism data (Es-2 locus) indicate that it is quite possible that the breeding-unit sizes and migration rates of natural populations considerably exceed those in which random drift would have an important effect on t allele frequencies. From these considerations it was concluded that random genetic drift itself does not serve as an adequate explanation for the low observed frequencies of lethal t alleles. Brief consideration was given to alternative hypotheses.

STUDIES OF NATURAL POPULATIONS OF MUS. II. POLYMORPHISM AT THE T LOCUS.

Studies of a number of natural populations of the house mouse, Mus musculus, by Dunn and his collaborators (Dunn and Morgan, 1953; Dunn, 1955; Dunn and Suckling, 1956; Dunn, Beasley, and Tinker, 1960; Anderson, 1964) have revealed the existence of a widespread polymorphism at the T locus. This polymorphism consists of a series of alleles which are concerned with the development of axial structures in the caudal region of the mouse. These alleles, the t-alleles, are divided into two major groups depending on their effect in the homozygous condition. Some of the t-alleles, when present in the homozygous condition (t/t) result in prenatal mortality and are described as lethals. Other t-alleles in the homozygous condition cause only male sterility, all homozygotes being viable. In view of the selective disadvantage of the t-alleles, their ubiquity is unexpected. The major force keeping these alleles in natural populations is an abnormal transmission ratio which favors the t-bearing sperm of heterozygous (+/t) males (Dunn, 1957). Since polymorphism at the T locus is widespread in natural populations of Mus musculus, an examination of the t-alleles was included in this study to support conclusions about breeding structure reached from data on other loci (Petras, 1965, 1967). In the process, information was obtained about: (1) the frequency of t-alleles in wild populations, (2) the fluctua-

A Genetic and Molecular Basis for Heterosis in Arabidopsis and Drosophila

Experimental evidence is consistent in showing that a considerable proportion of the heterotic expression for a character is a function of the temperature. There is relatively little heterosis at temperatures below the growth optimum, but as the temperature is increased above the optimum, so also is the heterosis. This is so for growth rate in Arabidopsis; and in Drosophila for frequency of heterozygous chromosomes, for wing and thorax length, for body weight and for various components of fitness (larval survival, total viability, egg production, longevity). The temperature-heterosis relation also holds for fluctuating temperatures (larval survival in Drosophila) and for high temperature shocks (growth rate in Arabidopsis and survival in Drosophila). The genetic basis for this phenomenon can be found in the common occurrence of high temperature sensitive alleles in natural populations. Such alleles have been demonstrated in Arabidopsis and in a large number of microorganisms. Different strains of these organisms possess different series of temperature unstable alleles. These alleles constitute a burden unavoidable by the organism because they are a frequent consequence of mutation and are relatively resistant to natural selection. The fraction of temperature sensitive alleles produced by forward mutation probably exceeds one-tenth of the total mutations. Many of the loci whose activity is restored by back-mutation are unstable to high temperature. The molecular basis of temperature sensitive mutants rests on the production of unstable enzymes by the affected loci. Such enzymes are usually unstable at any temperature, but the high thermal coefficient for inactivation leads to mutant expression only at elevated temperatures. It is suggested that mutational alteration of the amino acids of an enzyme, other than those comprising the catalytic center, sensitizes the enzyme to thermal denaturation. This interpretation of heterosis is intended only for organisms whose internal temperature adjusts to the environmental temperature.

INCOMPATIBILITY IN FLOWERING PLANTS

Summary1. Incompatibility is a physiological mechanism which enforces outbreeding. It is widespread throughout the families of flowering plants. There are two main types: (i) Heteromorphic. This is associated with differences in floral structure; distylic with two types of flower, thrum having a short style and high anthers, pin having a long style and low anthers; tristylic with three types of flowers, with long, mid and short styles. (2) Homomorphic, in which there are no floral differences.2. Heteromorphic incompatibility is associated with six contrasting pairs of characters; these are normally inherited as a single unit. The genetic control is by one gene, S, with two alleles in distyly, and by two genes each with two alleles in tristyly. The incompatibility reaction of the pollen is sporophytically and not gametophytically determined. Short style is always dominant to long. Major modifying genes and polygenes which affect the expression of the S gene are present. In Primula hortensis abnormal plants exist in which the complex of characters has been broken down, presumably by abnormal crossing‐over between subunits of which the S gene must be composed. Both the morphological and the genetic determination restrict the number of different types, and hence of alleles.3. Homomorphic incompatibility in many species is controlled by a single gene with a large number of alleles, Sj^Sg.Sn. Pollen is unable to grow in a stylewhich has the same allele as the pollen. Unlike heteromorphic incompatibility the pollen reaction is gametophytically determined and the two alleles in the style are independent. In some species, most of which are polyploids, the simple S type of control does not work without modifications.4. In a new tetraploid incompatibility breaks down and frequently the tetraploids are fully self‐compatible. This breakdown is due to interactions between two different alleles in diploid pollen grains. With some alleles the interaction is competitive so that neither allele functions. This results in self‐compatibility. With other alleles, one may be dominant to the other, thus retaining the self‐incompatibility of the diploid plant.5. Incompatibility is due in some species to the failure of pollen germination, in others to the pollen tubes failing to penetrate the full length of the style. Style‐grafting and temperature experiments indicate that the stoppage of pollen‐tube growth is due to a reaction between a substance in the pollen tube and a complementary substance in the style, the pollen and style substances being reactively different for each allele. The reaction blocks some process necessary for tube growth and is irreversible.6. Incompatibility in species crosses and their derivatives shows that species without an incompatibility system have polygenic modifiers which weaken the action of the S gene. From Nicotiana and Petunia hybrids there is evidence that the S gene can have an additional effect of inhibiting pollen from a self‐compatible species.7. The spontaneous mutation rate of the S gene in Oenothera and Prunus, including all changes, is of the order of one in a million pollen grains, but no mutations to a fully operative incompatibility allele have been found in seventy million tested grains.Mutant alleles produced by X‐rays are all of a type which have their pollen‐controlling activity destroyed but their stylar activity unimpaired. The X‐ray results and the evidence from polyploidy and interspecific hybrids all point to a dual structure of the S gene. The solution of the problem of the extremely low mutation rate with the high number of alleles in natural populations may throw light on the origin of new genie material.