Origin of genetic variation: regulation of genetic recombination in the higher organisms - a theory

Increase in the rate of recombinants in tomato (Lycopersicon esculentum L.) after in vitro regeneration

Modification to the cross-over (C. O.) rate of tomato (Lycopersicon esculentum) was attempted by using in vitro plant regeneration. F1 hybrids with the same genetical homozygous background were compared at two loci: "bs-ms32" on chromosome I, and "aa-d" on chromosome II. For each, the genetic distance separating the two markers was about 20 to 30 map units. One cotyledon of each F2 hybrid seedling was used as in vitro tissue culture material, while the rest of the plantlet was grown as a control. Recombination rates of the selfed progenies from each regenerated and matched control couple were compared. For the first set of markers 59,000 seeds were analysed (5 controls' and 7 regenerated progenies), and for the second, 11,000 (5 controls' and 8 regenerated progenies). There were significant increases in the genetic distance between markers in about half the regenerated individuals. For the first set the increases ranged from 6.07 to 6.91 units out of a control distance of the 19.84 to 25.65, corresponding to lengthenings of 30.59 to 35.29%. For the second set they ranged from 4.92 to 6.04 out of a control distance of 25.05 to 26.57, representing increases of 19.64 to 22.75%. Such a phenomenon can be important either from a fundamental or practical viewpoint, regarding selection efficiency in plants, and potential for gene reassortment.

Ecological aspects of the recombination problem

Some consequences of the effect of environmental factors on the recombination system are dealt with in this paper. There are two components involved - the system of individual adaptation (F-system) and the genetic system of population adaptation (R-system). Their interaction offers an optimum interrelation of immediate adaptivity and genetic flexibility within the population. Familiar data on the evolution of recombination control systems are considered in connection with the problems of induced broadening of genotypic variation spectrum with a view to selection.A notion of combined recombination rate and spectrum control is introduced here: disturbance control-due to direct influence of environmental factors on R-system (including meiotic processes), negative feedback control - due to dominance-recessiveness relationship between rec-loci alleles of the "fine" control system, and to the dependence of the R-system on the F-system norms of reaction to environment variation. The problem of dependence of the recombination spectrum on environmental factors has been considered and the hypothesis of a possible mechanism of such a dependence suggested.

Genetic recombination in mammalian somatic cells: a brief note

Crossing over in somatic cells is a likely source of genotypic modification observed in ageing and in carcinogenesis. The possible evolutionary significance of this mechanism is discussed.

Generation or multiple genetic specificities: origin of genetic polymorphism through gene regulation

Based on results of mutation studies in the fungus Schizophyllum commune, a new mechanism of the origin of genetic polymorphism is proposed. This may explain the intractable problems of the rise of multiple allelism controlling incompatibility in plants and the wide array of antibody diversity controlling immunity reaction in animals.

Investigations on inheritance of quantitative characters in animals by gene markers II. Expected effects

Monogenic characters mark chromosome sections and therefore allow estimation of the substitution effects of genes located in them for a quantitative character. From assumptions for higher organisms, the expected effects of a quantitative trait are deduced. The derivations aim to produce comparison values for subsequent empirical investigations in domestic mammals.

Improved estimate of the Gm-Pi linkage

The Gm-Piota linkage group is firmly established. A heterogeneity of recombination fraction amongst males of different Piota types has now become very likely. The major differences seem to be between the Piota (z) and other alleles of the Pi system. A chromosomal deletion, inversion or a regulatory (rec) locus in linkage disequilibrium with the Piota locus offer possible explanations.

Elimination of heterozygosity and efficiency of genetic systems

One of the significant observations of recent years in the field of population genetics, highlighted by electrophoretic isozyme studies, is the presence of considerable heterozygosity within experimental and natural populations of highly inbred plants. This is found to be a general phenomenon, and is attributed to heterozygote advantage. In "Parthenogenetic Diploidy", where an organism develops without fertilization after doubling of the haploid egg nucleus, genetic heterozygosity is abolished altogether. Parthenogenetic diploidy, therefore, offers an excellent opportunity to examine the relevance of heterozygote advantage to the efficiency of genetic systems and maintenance of specific populations. In this paper, based on the study of comparative incompatibility behaviour in parthenogenetic diploids and parent plants, a hypothesis is proposed explaining the significance of persistent heterozygosity in inbred populations.The role of heterozygosity in genetic systems can be long-term, evolutionary, through segregation and recombination, termed here SEGREGATIONAL heterozygosity; or immediate, developmental, through allelic "co-action" or interaction, INTEGRATED heterozygosity. It is proposed here that a certain degree of genetic heterozygosity of the INTEGRATED type is incorporated in the regulatory polygenic components of various genetic systems involved in an organism, and may be essential for the normal development characterising a physiological system, an ecotype, a species, or a higher group. INTEGRATED heterozygosity is effective in overcoming the barriers of limited threshold regimes in physiological systems, and hence is particularly significant in extending the inherent plasticity in the physiological expression of genetic systems.