Evolutionary Consequences of Restricted Gene Flow in a Marine Copepod

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Burton 9409066 The processes by which populations of organisms differentiate genetically and physiologically, achieve reproduction isolation,and ultimately undergo speciation are poorly understood. Using the intertidal copepod Tigriopus californicus as model system, levels of genetic differentialtion and reproductive isolation will be determined for populations in two geographic locations of specific interest: (1) the region around Point Conception (west of Santa Barbara, California) and (2) the north central coast of Baja California. The former region spans a geographic discontinuity in the historic distribution of the species. The latter region extends across the central coast of Baja, where a reproductively isolated population of Tigriopus has recently been found. Analysis of a series of populations in this region will span what appears to be a case of incipient speciation. Comparisons will be made of levels of allozyme differentiation and DNA sequence divergence across the two study regions- one that is marked by reproductive isolation and one that is not so marked - to gain insight into the consequence of restricted gene flow in Tigriopus. In addition to these descriptive studies, analysis of the function of cytochrome c oxidase (CO, EC 1.9.3.1) activity in interpopulation hybrids will be initiated. Because CO is composed of multiple nuclear and mitochondrially- encoded subunits, CO activity may be particularly sensitive to intergenomic functional integration (coadaptation). Analysis of CO enzymatic activity in control and back-crossed lines will be used to test whether normal patterns of inheritance of CO activity are perturbed by cytonuclear interaction in interpopulation in crosses. Several basic and applied issues concerning analysis of population structure will be addressed by the data. (1) What are the relationships among mtDNA sequence variation, nucDNA sequence variation, allozyme variation and reproductive compatibility highly subdivided populations? (2) Wh at are the relative sensitivities of DNA sequencing and allozyme studies for detecting population structure? (3) Are mtDNA haplotypes within a species functionally equivalent as is commonly assumed in phylogeographic studies, or have specific cytonuclear coadapted complexes evolved within geographically separated populations? (4) To what extent are biogeographic boundaries reflected in the genetic structure of species that span the boundaries? Issues (1-3) are important in developing methodologies for analysis of biodiversity in marine and terrestrial systems, while (4) is particularly relevant to assessing the biological impact of global climatic change.