Burton: Allozyme versus Dna Sequence Data

The genetic structure of natural populations of marine organisms is frequently inferred from the distribution of alleles at gene loci. Until recently, most investigations relied entirely on protein electrophoretic techniques, with particular emphasis on polymorphic enzyme-coding gene loci (allozyme loci). Over the past few years, increasing use has been made of molecular techniques, including methods that allow the construction of gene genealogies. These later methods provide powerful insight into the evolutionary history of genetic variation and, under some conditions, provide valuable information concerning population structure. This paper compares results of both allozyme and DNA sequence studies for a set of populations of the intertidal copepod Tt@iopus cul$ofovnicus along the California coast. The comparisons show that: (1) Allozynie frequencies distinguish almost all the study populations, whereas genealogies of the sampled DNA sequences do not. (2) Both nuclear and mitochondrial DNA sequences reveal strong population differentiation between central and southern California populations that is not apparent in the allozyme frequencies. (3) Allozymes and DNA sequences are not entirely concordant in the picture they present of population relationships. (4) The most complete analyses of population structure will require multiple genetic techniques. RESUMEN La estructura genktica de poblaciones naturales de organismos se infiere a menudo a partir de la distribuci6n de alelos en loci de genes. Hasta hace algunos aiios la mayoria de 10s estudios se basaban en tkcnicas de electroforksis de proteinas, con knfasis en loci de genes polimorfos que codifican enzinias (loci alocimos). En aiios recientes se ha incrementado el us0 de ttcnicas moleculares, incluyendo el us0 de mktodos que permiten construir genealogias genkticas. Estos mktodos ofrecen una perspectiva profunda de la historia evolutiva de la variacibn genktica; en ciertas circunstancias, estos mktodos proveen informacibn importante de la estructura de la poblaci6n. Este articulo compara 10s resultados obtenidos de alocimos y secuencias de ADN de un conjunto de poblaciones del coptpodo intermareal T&riopus culgofovnicus a lo largo de la costa de California. Las comparaciones produjeron varios resultados: (1) L a fiecuencias de alocimos distinguieron a casi todas las poblaciones estudiadas, mientras que las genealogias de las secuencias de ADN no las demarcb. (2) Tanto las secuencias del ADN de las mitocondrias como las de 10s nticleos revelaron una marcada diferenciacibn poblacional entre California Sur y Central, mas este resultado no se repiti6 con las frecuencias de alocimos. (3) Los resultados de las secuencias de ADN y 10s alocimos no concuerdan en su totalidad en el esquema de las relaciones poblacionales. (4) Los anilisis mas completos de la estructura de la poblacibn requeririn mtiltiples tkcnicas gentticas. INTRODUCTION Attempts to understand the genetic structure of marine invertebrate populations have long been hampered by our inability to directly track the dispersal of larval life stages that frequently spend substantial lengths of time in the plankton. Although current patterns and other physical factors may restrict or promote particular routes of dispersal, the oceans often appear to be barrier-free, and long-distance dispersal is clearly possible (e.g., Scheltema 1986). Despite this extensive potential for gene flow, population differentiation among marine invertebrates has now been widely documented through the use of biochemical and molecular genetic techniques (Burton 1983; Hedgecock 19%; Palumbi 1992). In particular, analyses of electrophoretically detected enzyme polymorphisnis have made major contributions to our understanding of gene flow and recruitment in a diversity of marine species over the past two decades. Several long-recognized problems arise from the use of allelic isozymes (allozymes) for the analysis of population structure. First, although protein electrophoresis allows the screening of many individuals, populations, and gene loci for analysis of population structure, its resolution is limited in that it detects only a subset of existing variants at a gene locus. Thus any putative allele may consist of multiple alleles that are indistinguishable, and differences between populations may frequently be underestimated. Second, as genetically different forms of the same enzyme, allozymes may differ in functional properties. Therefore, frequencies of alleles not only reflect patterns of gene flow and random genetic drift, but they may also result from the action of natural selection.