Large Genetic Distances Associated with Little Morphological Variation in Polycelis coronata and Dugesia tigrina (Planaria)

Abstract

Nixon, S. E., and R. J. Taylor (Department of Zoology, Arizona State University, Tempe, Arizona 85281 and Department of Biology, Western Washington State College, Bellingham, Washington 98225) 1977. Large genetic distances associated with little morphological variation in Polycelis coronata and Dugesia tigrina (Planaria). Syst. Zool. 26:152-164.-Genetic similarities among twenty-two populations of Polycelis coronata and one population of Dugesia tigrina (Planaria) were determined from electrophoretically obtained protein data. Similarity values between populations of Polycelis were lower than expected, being significantly below values reported for many vertebrate and invertebrate species. This difference is partially explained by various ecological and-behavioral isolating mechanisms. Lack of morphological variation between populations which does not correspond with the biochemical divergence may imply different rates of evolution at these two levels. Evolutionary divergence times based on the protein data are used as possible indices of the migratory pattern of Polycelis in the Pacific Northwest and correlate with the theory of land bridges between North America and Asia. [Planaria; electrophoresis; genetic similarity; phenograms; land bridges.] The use of electrophoresis in determining protein similarity between groups of organisms has proven to be an extremely useful tool in the study of population genetics. It is a method which can be used with facility and repeatability -in measuring the degree of genetic heterozygosity and polymorphism in natural populations and has become a valuable systematic aid. Reports of the use of electrophoretic data to estimate the degree of genetic similarity between populations, species, or higher taxonomic levels have been summarized by Avise (1974). Recently, protein similarities calculated from electrophoretic analyses has been used to determine genetic distances and evolutionary divergence times among species and groups of Drosophila (Nei, 1971; Lakovaara et al., 1972), the fish genus Bathygobius (Gorman et al., 1976), the amphisbaenian genus Bipes (Kim et al., 1976), four genera of starfish (Ayala et al., 1975), nine genera of minnows (Avise and Ayala, 1976), and lizards of the genus Anolis (Yang et al., 1974; Gorman and Kim, 1976). We have used a similar approach with 22 populations of Polycelis coronata (Girard) and one population of Dugesia tigrina (Girard), the two most common triclad flatworms in the state of Washington. Polycelis coronata is a common planarian species of western North America, extending from the Black Hills of South Dakota westward to the Pacific Coast. It is the most common freshwater flatworm in Washington and has an extremely wide geographical distribution in the state (Nixon, -1975). It is distinguished from other species of Polycelis primarily on the basis of sexual characteristics (such as morphological differences in the copulatory organs), albeit sexual maturity is apparently seldom achieved in natural populations. Because of this and the virtual lack of morphological variability among geographically separated populations, electrophoretic analyses are especially applicable to systematic studies of representative flatworms. Polycelis is restricted to fairly cold streams and springs, usually at high altitudes (above 650 meters), and appears to be stenothermic, tolerating temperatures between 40 and 140 C (Nixon, 1977). In order for individuals to migrate between springs, they would have to travel considerable distances downstream, there encountering warmer