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The genetic consequences of contrasting breeding systems in Plectritis (Valerianaceae) Layton, Charles Robert


This study describes the consequences of contrasting breeding systems on levels and organization of isozyme variation in two closely related taxa, Pleotvitis oongesta (Lindl.) D.C. and P. bvaahystemon F. £ M. (Valerianaceae). Both taxa are self-compatible, herbaceous winter annuals which occur along the Pacific coast from British Columbia to California. Nine enzyme systems were electrophoretically surveyed and fifteen loci identified in Pleotvitis. Genetic models were proposed to explain the observed variation. These models describe the multi-locus organization, sub-unit structure and allelic variation within each system. The models were tested with crosses and segregation analyses. When progeny classes were sufficiently large, segregations were analyzed using the chi-square statistic. Although P. congesta and P. brachystemon differed in their allelic complements, the data suggest that the multi-locus organization of the enzyme systems surveyed are identical in both species. Outcrossing rates were calculated for nine populations of P. brachystemon and fifteen populations of P. congesta. Their mean outcrossing rates were 2.4± 0.8% and 70.2± 4.8%, respectively, and the difference between these means is highly significant (P≤0.01). A number of genetic parameters were measured within each taxon to assess the effect that breeding system has on population structure. As measured by all parameters, the selfer maintains significantly less variation within populations than the outcrosser although their detected levels of total variation (H[sub t]) are not significantly different. These two species are ideal for comparison because: (i) they are closely related and have a large proportion of their alleles in common, (ii) the multi-locus organization of the enzyme systems studied is homologous and (iii) they have similar life-cycle strategies and habitat requirements. It is not uncommon, particularly on Vancouver Island, to find the two species growing sympatrically (i.e. in mixed populations). When the influence of site variation was controlled by comparing only sympatric populations of the two species, all genetic differences remained significant. The reduced level of variation within populations of the selfer was reflected in the F- statistics and in the analysis of gene diversity. On the average, P. congesta populations contained 85% of the variation detected within the species. Only 36% of the variation detected in P. brachystemon was maintained within local populations. In the selfer, the reduced level of genetic variation within populations was accompanied by increased populational differentiation. Preliminary data suggest that differentiation, particularly in the selfer, can occur over short distances. However, whether this represents micro-geographic differentiation or drift in small reproductively isolated populations is problematic. In this taxon, allele frequencies and single locus heterozygosities fluctuated widely and different alleles were often fixed in adjacent populations. In P. congesta, allele frequencies and heterozygosities also fluctuated among populations but not with the amplitude observed in P. brachystemon. There appears to be no relationship between genetic distance and geographic distance. This implies that gene flow between populations does not significantly influence allele frequencies. No evidence was found to suggest that the observed isozyme polymorphisms are selectively maintained. However, co-ordinated gene complexes and micro-habitat selection would probably not have been detected. It is concluded that the observed differences in the population structure of P. brachystemon and P. congesta is primarily the result of their contrasting breeding systems.

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