Differentiating between clonal growth and limited gene flow using spatial autocorrelation of microsatellites

In plant populations alleles often deviate from a random distribution and r eveal positive autocorrelation at short distances. In species with both clo nal and sexual reproduction, such clustering may be because ramets of the s ame genet were sampled at nearby locations. Alternatively, clustering may b e the result of limited gene flow through pollen or seeds (isolation-by-dis tance). Were, we modify a conventional spatial autocorrelation analysis usi ng the join-count statistic in order to differentiate between these two cau ses of genetic structure. We examined the distribution of seven microsatell ite loci representing 37 alleles in a 20 x 80 m plot of a perennial populat ion of eelgrass Zostera marina L. In analysing join-counts between all like genotypes we found significant genetic autocorrelation among ramets at dis tances between 1 and 7 m (P < 0.001). We then excluded joins between clonem ates which were identified from the expected likelihood of their seven-locu s genotypes. Without joins within genets, no autocorrelation was evident, i ndicating that most of the significant genetic clustering was caused by clo nal spread. At distances up to 27 m, alleles were distributed at random, in dicating a panmictic population at this spatial scale. These results illust rate the need for an a priori estimation of genet-ramet structure in clonal ly reproducing plants in order to avoid erroneous inferences about putative gene flow at various spatial scales.