A NARROW HYBRID ZONE BETWEEN CLOSELY RELATED CRICKET SPECIES.

Abstract

Our understanding of speciation events derives in large part from observations of closely related species or of differentiated populations that are thought to be incipient species. Such observations are particularly useful if they allow us to identify critical genetic changes that result in intrinsic barriers to gene exchange and to distinguish these from changes that simply accompany speciation (and might well occur in its absence). One approach is to examine interactions between closely related species or presumed incipient species where they occur sympatrically. With appropriate laboratory and/or field manipulations it may be possible to isolate the effects on degree of reproductive isolation of particular morphological, developmental or behavioral differences and then proceed to examine the genetic basis for such differences. This paper is the first in a series in which we examine the nature of barriers to gene exchange between egg-overwintering, fall adult field crickets (genus Gryllus). North American field crickets provide excellent material for evolutionary studies (Alexander, 1968; Harrison, 1978). Despite the existence of considerable morphological variation within this group, orthopteran taxonomists in the early part of this century concluded that Gryllus in North America consisted of a single, highly variable species (Lutz, 1908; Rehn and Hebard, 1915). Subsequent work, using calling song, life cycle and habitat as distinguishing characters, established the existence of at least eight distinct species in eastern North America alone (Fulton, 1952; Alexander, 1957; Alexander and Bigelow, 1960; Alexander and Walker, 1962; Walker, 1974). Here we are concerned with two of these species-the very closely related pair, G. pennsylvanicus and G. firmus. Gryllus pennsylvanicus occurs throughout the northeastern United States and extends south in the mountains to North Carolina and Georgia (Alexander, 1968). Throughout its range this species is univoltine, overwintering in the egg stage with adults appearing in late summer or early fall. Gryllus firmus, the 'beach cricket' (Fulton, 1952), is found along the Gulf Coast of the United States and along the Atlantic Coast as least as far north as the eastern shore of Maryland, where it is reported to occur together with G. pennsylvanicus (Alexander, 1957, 1968). In the northern part of the range the life cycle of G. firmus is nearly identical to that of G. pennsylvanicus. Further south adults of G. firmus appear in spring as well as fall (North Carolina) or throughout the year (Florida) and females lay both diapause and non-diapause eggs (Fulton, 1952; Alexander, 1968; Walker, 1981). Although populations of G. firmus and G. pennsylvanicus are morphologically distinct, no single morphological character is clearly diagnostic. The most reliable character for distinguishing the species appears to be ovipositor length in females (Alexander, 1957). In males, the number of teeth on the stridulatory file is greater in G. firmus (Rakshpal, 1960; Alexander and Walker, 1962; Nickle and Walker, 1974). In both sexes, G. firmus tends to be