Reference: Biol. Bull. 169: 638-651. (December, 1985) GENETIC BASIS OF SHELL VARIATION IN THAIS EMARGINATA (PROSOBRANCHIA, MURICACEA). I. BANDING IN POPULATIONS FROM VANCOUVER ISLAND A. RICHARD PALMER Department of Zoology, University of Alberta, Edmonton, Alberta T6G 2E9, and Bamfield Marine Station, Bamfield, British Columbia VOR 1BO Canada ABSTRACT Thais (or Nucelld) emarginata, a rocky intertidal gastropod inhabiting the Pacific coast of North America, is polymorphic for shell banding in nearly all habitats in which it is found. Two generations of breeding revealed that this polymorphism has a simple genetic basis. Banding of the outer shell was inherited as if controlled by a single autosomal locus (OB) with two alleles (OB B = banded and OB U = unbanded), with banding dominant. Banding also assorted independently of the major shell color locus (OC), thus reinforcing an earlier conclusion that shell traits in this species are not tightly linked in a 'super gene' as in many terrestrial pulmonates. The clarity of banding, however, was found to depend on other genes or alleles influencing pigment intensity; individuals carrying these alleles at the banding or other loci exhibited much less pronounced banding or in some cases a complete loss of pigment in the outer shell. Most commonly, spiral bands appear as regularly spaced lines of pigment set against a largely unpigmented background (= white to pale grey ground color), but bands may also occur against other ground colors. Mechanistically, however, banding appears to result from regularly spaced zones of suppressed pigmentation in the outer shell. Preliminary distributional data revealed that the frequency of banded individuals in field populations increased with increasing wave exposure; however, the adaptive value of this polymorphism is not clear at present. INTRODUCTION Many species of marine, shallow-water, prosobranch gastropods exhibit intraspecific variation in shell pigmentation (Kincaid, 1957, 1964; Spight, 1976; Clarke, 1978). The rather conspicuous variation in some species has prompted numerous studies of the correlation between morph-frequencies and environmental conditions (Colton, 1922;Pelseneer, 1935; Moore, 1936; Fischer-Piette etal, 1963; Daguzan, 1968; Safriel, 1969; Berry and Crothers, 1974; Heller, 1975; Pettitt, 1975; Hoagland, 1977;Osborne, 1977; Davis, 1980; Bowman, 1981; Reimchen, 1981; Smith, 1981). Surprisingly few have examined experimentally the adaptive value of variation in shell pigmentation (Giesel, 1970; Reimchen, 1979; see also Mercuric et al, 1985). In addition, and in contrast to the situation for aquatic and terrestrial pulmonates (reviewed by Murray, 1975), little is known about the genetic basis of this variation (but see Cole, 1975; Palmer, 1984a). I report here the results from two generations of breeding, including backcrosses: that variation in shell banding in populations of the rocky intertidal gastropod Thais (or Nucella) emarginata is controlled predominantly by a pair of alleles at a single autosomal locus. Knowledge of the genetic basis of shell banding Received 19 April 1985; accepted 23 September 1985. 638
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