RIBOSOMAL (RDNA) VARIATION IN A DEEP-SEA HYDROTHERMAL VENT POLYCHAETE, ALVINELLA-POMPEJANA, FROM 13-DEGREES-N ON THE EAST PACIFIC RISE

The rDNA repeat-unit of the vent polychaete Alvinella pompejana was in vestigated using restriction analysis. Mapping revealed evidence of rD NA polymorphism within and between individuals which was due to indivi dual restriction site variation and sequence rearrangements involving spacer regions. The size of the repeat unit was 10.5 kb with virtually no evidence of length variation. Sequence inversions indicated the pr esence of two spatially-distinct subfamilies of repeats, probably on d ifferent chromosome pairs. Animals from contrasting vent habitats with respect to age and chemical emissions (young vs old chimneys and whit e vs black smokers) from within the 13 degrees N/EPR (East Pacific Ris e) vent sector were analysed for evidence of population differentiatio n. Based on individual restriction site variation, average F-ST estima tes across neighbouring populations were in the region of similar to 0 .05 and differed significantly from zero. This level of genetic differ entiation is comparable to values reported previously for allozymes. S patial and temporal allelic frequency variances estimated from pairwis e combinations (i.e. s(2)S and s(2)T) strongly suggested that differen ces in allelic frequency were the result of repeated extinction/recolo nization events associated with the vent instability. Estimates of the effective population size derived from standardized temporal allelic frequency variances Fks were very low compared to actual population si ze indicating great temporal fluctuations in the former. Theoretically , such an effective population size is not sufficient to maintain the observed level of polymorphism within the 13 degrees N/EPR vent sector . Results are therefore consistent with a 'propagule' colonization-typ e model in which extinction/recolonization rates are high. In Alvinell a, planktonic larval dispersal appears sufficient to overcome any gene tic differentiation resulting from drift, but these findings also indi cate that propagules may only be capable of dispersing a few tens of k ilometres per generation.