THE EVOLVING GENETIC HISTORY OF A POPULATION OF LATHYRUS-SYLVESTRIS -EVIDENCE FROM TEMPORAL AND SPATIAL GENETIC-STRUCTURE

We analyze patterns of genetic microdifferentiation within a natural p opulation of Lathyrus sylvestris, a perennial herb with both sexual re production and clonal growth. In a population from the northern foothi lls of the Pyrenees in southwestern France, a combined demographic and genetic investigation enabled the study not only of spatial genetic s tructure of the population, but also of the history of the population' s spatial genetic structure over time. Excavation of all individuals a llowed identification of clonemates. Age of each individual was determ ined by counting annual growth rings in the taproot, a method tested w ith individuals of known age planted in experimental gardens. Each ind ividual was mapped, and genotypes of all individuals were determined u sing allozyme markers for a number of polymorphic loci. Distribution p atterns and spatial genetic structure, both for all individuals and fo r different age classes, were analyzed using spatial autocorrelation s tatistics (Geary's Index, Moran's Index). Patterns of gene flow within the population were also studied using F-statistics and tests for ran dom associations of alleles. Because age, allele frequencies, and loca tion were known for each individual, it was possible to study how spat ial genetic structure changed over lime. Results from all these divers e approaches are consistent with one another, and clearly show the fol lowing: (1) founder effects, with the study transect being first colon ized by individuals at either end of the transect that were homozygous for different alleles at one marker locus; (2) a difference in spatia l distribution of individuals originated from sexual reproduction (see dlings) and from clonal growth (connected individuals); (3) restricted gene flow, due to inbreeding among related, clumped individuals; and (4) increase in heterozygote deficit within the youngest cohort of ind ividuals. The results indicate that genetic differentiation in time wa s much less marked than differentiation in space. Nevertheless, the re sults revealed that the studied population is experiencing demographic and genetic variation in time, suggesting that it is not at equilibri um. On the one hand, spatial structuring is becoming less marked due t o the recombination of founder genotypes; on the other hand, as establ ishment of new individuals increases, a new spatial structure emerges due to mating between relatives.