a1 Marine Biological Association of the United Kingdom, The Laboratory, Citadel Hill, Plymouth, PL1 2PB.
a2 Plymouth Marine Laboratory, Citadel Hill, Plymouth, PL1 2PB.
a3 IFREMER, Centre de Brest, DRO/EP, BP 70, 29280, Plouzané, France.
The rDNA repeat-unit of the vent polychaete Alvinella pompejana was investigated using restriction analysis. Mapping revealed evidence of rDNA polymorphism within and between individuals which was due to individual 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 presence of two spatially-distinct subfamilies of repeats, probably on different chromosome pairs. Animals from contrasting vent habitats with respect to age and chemical emissions (young vs old chimneys and white vs black smokers) from within the 13°N/EPR (East Pacific Rise) vent sector were analysed for evidence of population differentiation. Based on individual restriction site variation, average FST estimates across neighbouring populations were in the region of ~0.05 and differed significantly from zero. This level of genetic differentiation is comparable to values reported previously for allozymes. Spatial and temporal allelic frequency variances estimated from pairwise combinations (i.e. s2S and s2T) strongly suggested that differences in allelic frequency were the result of repeated extinction/recolonization 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 size 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 13N/EPR vent sector. Results are therefore consistent with a ‘propagule’ colonization-type model in which extinction/recolonization rates are high. In Alvinella, planktonic larval dispersal appears sufficient to overcome any genetic differentiation resulting from drift, but these findings also indicate that propagules may only be capable of dispersing a few tens of kilometres per generation.
p1 Present address: Laboratoire d'Ecophysiologie, CNRS UPR 9042, Station Biologique de Roscoff, BP 74, 29682, Roscoff Cedex, France