ESTIMATES OF GENE FLOW IN EICHHORNIA-PANICULATA (PONTEDERIACEAE) - EFFECTS OF RANGE SUBSTRUCTURE

Gene flow in the annual aquatic plant, Eichhornia paniculata, was infe rred from estimates of genetic differentiation at 24 isozyme loci amon g 44 populations from north-east Brazil. Population differentiation, e stimated as the correlation among genes of different individuals (phi) , was 0.45 (range among loci, 0.10-0.69). Based on Wright's island mod el, this heterogeneity would result from gene flow equivalent to 0.31 immigrants (Nm) entering each population per generation. The distribut ion of E. paniculata in north-eastern Brazil is geographically and gen etically subdivided, and therefore, the assumption that migrants are a random draw from all populations is likely to be violated for this ra nge-wide estimate of gene flow. We investigated the importance of rang e subdivision on indirect estimates of gene flow through computer simu lation and through a hierarchical analysis of F-ST and Nm in populatio ns of E. paniculata from northeastern Brazil. Simulations indicated th at estimates of Nm in the presence of range substructure consistently underestimated the actual values of gene flow. The degree to which Nm was underestimated increased in proportion to the magnitude of genetic differentiation among range subdivisions. In E. paniculata, northern and southern regions of the geographical range and local clusters with in regions were genetically differentiated (phi = 0.10, phi = 0.14, re spectively) and there was a strong negative relationship between Nm es timated for pairs of populations and the geographical distance between them. Average estimates of population differentiation decreased and g ene flow increased with successive reductions in spatial scale, from t he complete range sampled (Nm = 0.31), to regions (Nm = 0.44), to loca l areas within regions (Nm = 0.64), and to neighbouring population pai rs within local areas (Nm = 0.58). Similarity in estimates for neighbo uring pairs and local areas suggests that, below the spatial scale of local area, gene flow estimates are not influenced by range substructu re and can be considered to occur at random. Our results suggest that range substructure can have a substantial influence on gene flow estim ates, and that ecologically relevant rates are likely to be higher tha n those indicated by range-wide analyses in organisms with geographica l subdivision.