GENETIC VARIATION IN THE HORSESHOE CRAB (LIMULUS POLYPHEMUS), A PHYLOGENETIC "RELIC".

Abstract

To many early evolutionists, particularly those of mutationist school, existence of evolutionarily stable forms (living fossils or phylogenetic relics) posed difficult problem, and, even among contemporary biologists, feeling that a long-standing stability of organization seems antithetical to concept of may persist (Volpe, 1967:140). In early part of this century, it was popularly believed that stable or slowly evolving (bradytelic) lines lacked genetic variation necessary for more rapid evolution as result of having abnormally low mutation rates; as Mayr (1963) puts it, lack of variation in time and space was interpreted as reflection of genotypic stability. Largely because of influence of Simpson (1944), Stebbins (1944), Schmalhausen (1949), and Dobzhansky (1951), mutationists' interpretation of has now been all but abandoned by evolutionists in favor of view that principal factors controlling rates of evolution are to be found in organism-environment relationship. Additionally, Mayr (1963 :304) has suggested that phenotypic uniformity in space and time is due to highly perfected 'buffering system,' in other words, to genetic homeostasis. The ecological hypothesis is well expressed in Simpson's (1944:141) conclusions that the final and probably most fundamental factor in relationship is that bradytelic groups are so well adapted to particular, continuously available environment that almost any mutation occurring in them must be disadvantageous and that bradytely results from equilibrium of large breeding populations of animals specifically adapted to continuously available environment that is relatively invariable . Yet, as Mayr (1963) notes, there is no evidence to refute (or to support) hypothesis that results from loss of mutability. The recent development of electrophoretic techniques for demonstrating allelic variation at loci controlling enzymes and other proteins in natural populations (Hubby and Lewontin, 1966; Lewontin and Hubby, 1966) permits test of mutationists' thesis. The purpose of this study is to compare degrees of protein polymorphism and underlying genic heterozygosity in classic phylogenetic relic, horseshoe or king crab (Limulus polyphemus), and in several representatives of horotelic lines (those evolving at standard rates) for which data are already available. Evidence of level of genetic variation in Limulus similar to that in horotelic forms will support modern ecological and homeostatic interpretations of bradytely. But if mutationists' thesis is valid, Limulus should show an unusually low degree of polymorphism at its genetic loci. Limulus polyphemus is one of five living species of Subclass Xiphosura, group of marine arthropods usually placed with Subclass Eurypterida (extinct giant water scorpions) in Class Merostomata of Subphylum Chelicerata. Following period of adaptive radiation in early and middle Paleozoic, Merostomata became much less prominent after Permian. Several Mesozoic forms are known in Family Mesolimulidae, but by Tertiary fossil record is all but absent. There are presently three living genera, all belonging to Family Limulidae, only one of which, Tachypleus, is known from fossil record (Stormer,