NUCLEAR AND MITOCHONDRIAL GENE GENEALOGIES AND ALLOZYME POLYMORPHISM ACROSS A MAJOR PHYLOGEOGRAPHIC BREAK IN THE COPEPOD TIGRIOPUS-CALIFORNICUS

The genetic structure of natural populations is frequently inferred fr om geographic distributions of alleles at multiple gene loci. Surveys of allozyme polymorphisms in the tidepool copepod Tigriopus californic us have revealed sharp genetic differentiation of populations, indicat ing that gene flow among populations is highly restricted. Analysis of population structure in this species has now been extended to include nuclear and mitochondrial gene genealogies. DNA sequences of the mtDN A-encoded cytochrome-c oxidase subunit I gene from 21 isofemale lines derived from seven populations reveal a phylogeographic break between populations north and south of Point Conception, California, with sequ ence divergence across the break exceeding 18%, the highest level of m tDNA divergence yet reported among conspecific populations. Divergence between populations based on 22 sequences of the nuclear histone H1 g ene is geographically concordant with the mitochondrial sequences. In contrast with previously studied nuclear genes in other sexually repro ducing metazoans, the H1 gene genealogy from T. californicus shows no evidence of recombination. The apparent absence of intragenic recombin ants probably results from the persistent lack of gene flow among geog raphically separated populations, a conclusion strongly supported by a llozyme data and the mitochondrial gene genealogy. Despite strong popu lation differentiation at allozyme loci, the phylogeographic break ide ntified by the DNA sequences was not evident in the allozyme data.