Multiple CMS-restorer gene polymorphism in gynodioecious Plantago coronopus.

The mode of inheritance of the male sterility trait is crucial for understanding the evolutionary dynamics of the sexual system gynodioecy, which is the co-occurrence of female and hermaphrodite plants in natural populations. Both cytoplasmic (CMS) and nuclear (restorer) genes are known to be involved. Theoretical models usually assume a limited number of CMS genes with each a single restorer gene, while reality is more complex. In this study, it is shown that in the gynodioecious species Plantago coronopus two new CMS-restorer polymorphisms exist in addition to the two that were already known, which means four CMS-restorer systems at the species level. Furthermore, three CMS types were shown to co-occur within a single population. All new CMS types showed a multilocus system for male fertility restoration, in which both recessive and dominant restorer alleles occur. Our finding of more than two co-occurring CMS-restorer systems each with multiple restorer genes raises the question how this complex of male sterility systems is maintained in natural populations.

Variation in growth rate in a natural assemblage of unicellular green soil algae.

Unicellular, motile, phototropic green algae were extracted from soil samples taken at metre intervals along a 25-m transect in a wheat field. The vegetative growth of 61 randomly selected isolates (henceforth called spores) was measured in dark and light conditions, and at high and low nutrient concentrations in liquid media. The among-spore variance was dominated by a spore-by-environment interaction. The increase of among-spore variance with distance was detectable but slight, showing that most of the diversity found on the transect was present at a scale of metres. A mixture of all spores, constituting the founder assemblage, was propagated for 50-70 generations in three environments to study the sorting of variation initially present in the assemblage. Adaptation to the new environments was measured by improvement in growth relative to the founder assemblage. All three sets of lines became adapted to their selection environment, although the extent of the advance depended on the amount of among-spore variance in the founder assemblage. Selection for increased growth in the dark, or at high nutrient conditions in the light, caused the assemblage to evolve to the limit of the variation initially present, strongly reducing among-spore variance as a consequence. At low nutrient concentrations selection was less effective, and there was little reduction of the among-spore variance. The correlated responses of each group, again relative to the founder assemblage, were measured over the entire experimental range. Correlated responses were negative when performance in widely different (light and dark) environments was compared, but positive for more similar environments (high and low nutrient concentrations).

The dynamics of gynodioecy in Plantago lanceolata L. II. Mode of action and frequencies of restorer alleles.

Male fertility in Plantago lanceolata is controlled by the interaction of cytoplasmic and nuclear genes. Different cytoplasmic male sterility (CMS) types can be either male sterile or hermaphrodite, depending on the presence of nuclear restorer alleles. In three CMS types of P. lanceolata (CMSI, CMSIIa, and CMSIIb) the number of loci involved in male fertility restoration was determined. In each CMS type, male fertility was restored by multiple genes with either dominant or recessive action and capable either of restoring male fertility independently or in interaction with each other (epistasis). Restorer allele frequencies for CMSI, CMSIIa and CMSIIb were determined by crossing hermaphrodites with "standard" male steriles. Segregation of male steriles vs. non-male steriles was used to estimate overall restorer allele frequency. The frequency of restorer alleles was different for the CMS types: restorer alleles for CMSI were less frequent than for CMSIIa and CMSIIb. On the basis of the frequencies of male steriles and the CMS types an "expected" restorer allele frequency could be calculated. The correlation between estimated and expected restorer allele frequency was significant.

Genetics of male sterility in gynodioecious Plantago coronopus. I. Cytoplasmic variation.

Inheritance of male sterility was studied in the gynodioecious species Plantago coronopus using five plants and their descendants from an area of approximately 50 m2 in each of six locations. The crosses were planned to test for cytoplasmic inheritance of male sterility. In four locations significant differences between reciprocal crosses were observed. The progenies of these reciprocal crosses were used in a crossing scheme designed to test whether these reciprocal differences were caused by different cytoplasmic types between the plants. In all four locations, the existence of at least two cytoplasmic types could be shown. Moreover, the results of the crosses between locations showed that the same two cytoplasmic types were present in all four locations. We therefore argue that there is only limited cytoplasmic variation in P. coronopus. In each cytoplasmic type a series of intermediate sex forms occurred. A marked difference in restoration level existed between the two cytoplasmic types. Plants with cytoplasmic type 2 hardly segregated male steriles, in contrast to plants with cytoplasmic type 1.

Genetics of male sterility in gynodioecious Plantago coronopus. II. Nuclear genetic variation.

Inheritance of male sterility was studied in the gynodioecious species Plantago coronopus using five plants and their descendants from an area of approximately 50 m2 from each of four locations. In each location, crosses between these five plants yielded the entire array of possible sex phenotypes. Both nuclear and cytoplasmic genes were involved. Emphasis is placed on the nuclear (restorer) genetics of two cytoplasmic types. For both types, multiple interacting nuclear genes were demonstrated. These genes carried either dominant or recessive restorer alleles. The exact number of genes involved could not be determined, because different genetic models could be proposed for each location and no common genetic solution could be given. At least five genes, three with dominant and two with recessive restorer allele action, were involved with both cytoplasmic types. Segregation patterns of partially male sterile plants suggested that they are due to incomplete dominance at restorer loci. Restorer genes interact in different ways. In most instances models with independent restorer gene action were sufficient to explain the crossing results. However, for one case we propose a model with epistatic restorer gene action. There was a consistent difference in the segregation of male sterility between plants from the two cytoplasmic types. Hermaphrodites of cytoplasmic type 2 hardly segregated male steriles, in contrast to plants with cytoplasmic type 1.

Selection on reaction norms, genetic correlations and constraints.

Two approaches to the evolution of phenotypic plasticity in heterogeneous environments have recently been put forward. The first focuses on selection on the character expression within each environment; plasticity is seen as a by-product of local selection in various habitats. The second approach focuses on selection on the parameters of the response function of genotypes, and selection is thought to change the frequencies of 'plasticity' genes that affect the function. This paper discusses the relationship between the two approaches, with emphasis on applications. A method is described that allows switching from one approach to the other. It is argued that character state and reaction norm approaches, while to a large extent interchangeable, usually differ in the response function chosen. This choice, however, may strongly affect the biological interpretation. The methods outlined in this paper permit one to look at the data from different perspectives in order to avoid this danger.