Elements of the S-gene complex III. Chromosome fragments and naturally occurring S-gene mutations in Nicotiana bonariensis

Sexually localized expression of pseudo-self compatibility (PSC) in Petunia X hybrida Hort. : 1. Pollen inactivation

A naturally occurring, 100% PSC Petunia X hybrida Hort. plant was found which had normal stylar function, but lacked S allele activity in the pollen. Preliminary characterization showed the phenotype to be identical to that of a pollen part mutant as described by Lewis (1949). Linkage test results, using a plant with the pollen-inactivation PSC factor as male parent, agreed with earlier observations that crossing over between S and the factor was rare. However, when the plant bearing the factor was used as female, apparent recombinants were recovered, although they were all of one class. Unequal gamete competition based on inter-line incongruity could explain the failure to recover recombinants from the male parent.

'Hertwig Effect' in plants: induced parthenogenesis through the use of irradiated pollen

Studies of four combinations of Nicotiana involving four species, N. langsdorffii (N. l.), N. alata (N. a.), N. glutinosa (N. g.) and N. tabacum (N. t.), have shown that parthenogenetic haploid and diploid maternal individuals may arise with the use of male gametes (pollen) treated with high doses of ionising radiation in plants, similar to that found in animals ("Hertwig Effect'). At lower doses (10-20 Kr) rapidly diminishing numbers of seedlings were produced and many of these died soon after germination or before reaching maturity. In the intraspecific combination N. l. × N. l., viable seeds were produced only at the lower doses of 10 and 15 Kr. In the interspecific combination N. l. × N. a., at lower doses, all plants that came to bloom showed variable hybrid morphology. There were no plants resembling the female parent. In the combination N.t. × N.a., at lower doses there were rare surviving plants which were maternal dihaploid (1 plant out of 4 at 15 Kr) or tetraploid N. tabacum (all 5 plants at 20 Kr). All surviving plants at higher doses (50 and 100 Kr) were maternal tetraploids. In the combination N.t.×N.g., plants produced at lower than 20 Kr were almost all either aneuploid or triploid hybrids. Dihaploid, maternal N. tabacum plants appeared at 20 Kr and higher doses. After 50 Kr the large proportion of plants produced were maternal dihaploid or tetraploid N. tabacum.

Origin of complementary incompatibility systems in flowering plants

The complementary incompatibility system, characterized by co-operative control of a single S specificity by alleles of two or more distinct S genes, has raised interesting questions regarding the origin and evolutionary significance of this system. What were the factors which led to the appearance of the complementary system of self-incompatibility. Is complementary incompatibility a primary or secondary development?Lundqvist and Østerbye have suggested that the fundamental characteristic of this system - lack of dominance and competitive interactions between alleles of the same and different series - developed once, early in the evolution of angiosperms, at a stage when self-incompatibility was controlled polygenically. In one line of development, where two or more such incompatibility genes were strengthened by specific modifiers at the expense of the rest, co-operation among loci was favoured to promote increased interplant compatibility in the population. In this evolutionary line, allelic or intergenic interactions were excluded. In the other line of development, where only one incompatibility locus was strengthened, there was no need for such selection and alleles of this locus retained the property of allelic interaction in the pollen.In this article an alternative hypothesis has been proposed for the origin of complementary incompatibility. It is suggested that this type of incompatibility system, conforming with generally held views on the evolution of self-incompatibility systems, developed secondarily, and polyphyletically, after the breakdown of the original one-locus, multiallelic, gametophytic system. In the re-revolution of self-incompatibility through introgression with a related self-incompatible taxon, the essential action of the presumed physiologically integrated self-compatible complex led to the exclusion of allelic or intergenic interaction as a prerequisite for evolution of complementary control. According to this hypothesis, breakdown of the original self-incompatibility and re-evolution of self-incompatibility, in the manner suggested above, could have occurred many times in the evolution of angiosperms and such systems might therefore be expected to occur scattered among different phylogenetic lines.

Evolution of incompatibility systems in plants: Complementarity and the mating locus in flowering plants and fungi

The restriction of sexual pairing by a specificity gene is considered to be an ancient development in the plant kingdom. The diversity and general parallelism of incompatibility systems seen amongst the phyla at the present time can be rationalized in terms of the association of various derived forms of the ancestral specificity unit with differing spectra of accessory factors controlling sexual physiology in the different phyla. Sexual morphogenesis has become divided into distinct phases under the control of complementary genes. These phases are initiated by a regulatory system of "Co-ordinator genes" which control the order in which groups of morphogenetic genes are expressed during development. The entire sexual cycle will be completed only if all the complementary groups are activated in the appropriate sequence. The present article discusses essential features of the evolution of the breeding locus in different phyla. These features are consistent in themselves with the present data and are not dependent on the proposed ancient origin of the specificity gene.The above hypothesis throws light on the (1) evolution of the complex mating loci in flowering plants and fungi; (2) evolution of complementary incompatibility and heteromorphic incompatibility in flowering plants; (3) anomalous cross-compatibility behaviour of mutants in the fungus Schizophyllum commune; (4) nature of homothallism in higher fungi; (5) mode of origin of new functional self-incompatibility alleles; and (6) "homogenic" and "heterogenic" incompatibility.

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

Recent studies in the fungi, particularly Neurospora and Schizophyllum, have revealed a number of genetic features which, viewed in conjunction with earlier observations on other organisms, form a pattern, or model, which appears to be basic to the control of recombination in all eukaryotes, including higher organisms. It is assumed that the control is exercised on mechanisms that produce new alleles through recombination, as understood in broad terms and including such a likely phenomenon as gene conversion, which may or may not involve crossing-over, as well as equal and unequal crossing-over. The recombination may thus occur between alleles in either the homozygous or heterozygous condition. In the model, regulatory genes and breeding behaviour are integrated into one self-regulatory system controlling the production of new genetic variation.The model is based on the following five general features, largely substantiated by the results in Neurospora and Schizophyllum: 1) The frequency of recombination in a particular chromosomal region is controlled by specific regulatory genes (rec). 2) There may be a number of such specific, regulatory genes responsible for recombination in a given region. 3) A rec. locus may influence recombination in more than one region. 4) The regulatory genes have no specific physical relationship with the region(s) they control, and are usually located at random in the genome. 5) Of the allelic forms of the regulatory genes it is always the dominant gene which suppresses recombination and the recessive gene which increases recombination. The rec system is epistatic to other genetic elements jointly involved in the overall control of recombination in a specific region. It is suggested that usually the control of recombination in a given region is exercised, cumulatively, by the balance of the dominant and recessive genes of the specific rec loci in the organism. Outbreeding, with the associated high heterozygosity of the regulatory rec loci, virtually "switches off" recombination, producing few new variations. Inbreeding produces homozygosity of these loci, resulting in certain individuals which will have a considerable number of their regulatory loci in the homozygous recessive condition and in which recombination will be "switched on", producing new variation at a high frequency. Inbreeding is thus an integrated, evolutionary system of considerable importance, and is not a degenerate "dead end", as many investigators have previously thought.The model has another compensatory function in evolution. In major loci, or in an operon, where there are structural genes and closely linked operator genes, as exemplified by the S locus, there are indications that the present model is concerned with the regulation of both structural and operator genes. The consequences of the model in the two classes of genes, however, are in direct contrast to each other: High heterozygosity which is instrumental in switching "off" recombination, and which is therefore helpful in maintaining stability in the structural gene, is conducive to functional variation of the operator gene; and high homozygosity, which is instrumental in switching "on" recombination, and which is therefore helpful in producing variation in the structural gene, is conducive to the stability of the operator gene.This model of the control of genetic variation in a specific chromosomal region is significant in development as well as in evolution, and throws light on a number of hitherto "intractable" problems peculiar to the higher organisms. For example, the model is helpful in explaining: 1) the origin of new self-incompatibility alleles in the flowering plants; 2) the impressive speciation in the waif flora (and fauna) of the oceanic islands; 3) the presence of high genetic variability in inbreeding species of plants; 4) environmentally-induced heritable variation in certain plants; and 5) the genetic mechanism of antibody diversity in animals.