Comparative genetic structure of two co-occurring tree species, Maclura pomifera (Moraceae) and Gleditsia triacanthos (Leguminosae)

Maclura pomifera, an autotetraploid, and Gleditsia triacanthos, a diploid, are ecologically similar dioecious tree species that often co-occur in early successional habitats throughout the mid-western United States. We studied levels of genetic diversity and patterns of genetic structure for four polymorphic enzyme loci of M. pomifera and 16 polymorphic enzyme loci of G. triacanthos from a single population in eastern Kansas. Levels of expected heterozygosity were high for both species, averaging 0.725 for M. pomifera and 0.366 for G. triacanthos. Although genotypes for nearly all G. triacanthos loci were in Hardy-Weinberg frequencies, three of four M. pomifera loci deviated from equilibrium expectations. Two aspects of genetic structure were explored. First, the extent of clonal growth was estimated by comparing genotypes of stems within 50 G. triacanthos and 32 M. pomifera clumps. The great majority of clumps contained more than one genotype, and in many clumps, all stems were genetically unique. Secondly, as revealed by spatial autocorrelation analyses, genetic substructure was very local for both species, with significant positive autocorrelation occurring only within clumps of individuals or among near neighbours. We argue that this pattern of spatial structure for both species results from extremely local seed dispersal and establishment of individuals from the same multiseeded fruit