Variation in Population Sex Ratio and Mating Success of Asexual Lineages of Alsophila pometaria (Lepidoptera:Geometridae)

Populations of Alsophila pometaria (Harris) are largely gynogenetic and composed of asexual females that must mate with a conspecific male to reproduce. Within this asexual contingent, there are a variety of electrophoretically defined multilocus genotypes. In this study we found that populations differ in sex ratio. We also found that there is an indication of differential mating success among asexual genotypes in a population of A. pometaria. Gynogenesis is an unusual form of reproduction in which asexual females must mate before reproducing, but males make no genetic contribution to the offspring. Gynogenetic females of Alsophila pometaria (Harris), the fall cankerworm, pass on their genotype intact to progeny yet must mate with a conspecific male to reproduce (Mitter and Futuyma 1977). Electrophoretic surveys and breeding studies have shown that there are a variety of asexual genotypes (clones) within populations of this species (Mitter et al. 1979, Harshman 1982). Sexual reproduction is also present and sexual females are assumed to be the source of males upon which gynogenetic females depend (Mitter et al. 1979, Harshman 1982). The population sex ratio of the fall cankerworm is female-biased (Porter and Alden 1924, Balch 1938, Mitter et al. 1979), reflecting the preponderance of asexual reproduction. In this study we have surveyed local populations for variation in sex ratio. Male abundance is a critical parameter in gynogenetic populations (Kiester et al. 1981), particularly H A R S H M A N A N D F U T U Y M A , A N N A L S O F T H E E N T O M O L O G I C A L S O C I E T Y O F A M E R I C A 7 8 (1 9 8 5 ) 2 so where sex ratios are extreme and many females go unmated. We also studied whether or not there was differential mating success among the asexual lineages found in populations of the fall cankerworm. Gynogenetic species may often be composed of a variety of asexual genotypes (Vrijenhoek 1979, Mitter et al. 1979), and where males are in short supply, mating success could be an important component of fitness of asexual lineages. Materials and Methods Population Sex Ratio On Long Island, New York, eggs of the univoltine fall cankerworm hatch in late April and early May. Larvae are polyphagous and usually found feeding on the foliage of deciduous tress. Pupation occurs in the soil, where there is an obligate summer diapause. In the last week of May 1981, late-instar larvae were collected from seven sites by striking vegetation over a beating net. Samples were brought into the laboratory and larvae were fed oak leaves (Quercus coccinea and Quercus velutina) until they were ready to pupate (usually within a week). There was little mortality in the laboratory and prepupae were placed in a mixture of sterilized sand and potting soil. After 2 months the subterranean cocoons were unearthed and pupae extracted. Male and female pupae could be distinguished by differences in body shape, genital pore position, and wing development. It is difficult to devise an unbiased sampling procedure for adults because the females are wingless and the males fly. Consequently, it seems likely that our method of determining population sex ratio from collections of larvae is more accurate than counting or trapping adults in the field. All the collection sites were on Long Island (fig. 1). The north shore of the island generally has mesic habitat dominated by species of the red oak group (subgenus Erythrobalanus) and red maple (Acer rubrum). This includes the Graduate Biology and Ashley Schiff sites, which are on the Stony Brook campus of the State University of New York, and the Village Wood site, which is located several kilometers away in the village of Stony Brook. Belmont State Park and Southern State Parkway are near the southern shore of the middle of Long Island. The Belmont State Park site is mesic and characterized by trees of the red oak group and red maple. The Southern State Parkway site is drier and dominated by white oak (Quercus alba) and scrub oak (Q. ilicifolia). The remaining two locations are in the sandy outwash plain that makes up much of the southeastern portion of the island. One of these locations, Route 27, is almost entirely composed of scrub oak and stunted pitch pine (Pinus rigida). The other, called Low Planes, is similar but has several additional oak species. H A R S H M A N A N D F U T U Y M A , A N N A L S O F T H E E N T O M O L O G I C A L S O C I E T Y O F A M E R I C A 7 8 (1 9 8 5 ) 3 Figure 1. Alsophila pometaria collection sites on Long Island, New York. Mating Success Adults of the fall cankerworm emerge on Long Island in the late fall or early winter. Females are wingless and may be found on tree trunks, apparently releasing pheromone to attract males flying in the vicinity. Mating may last for several hours, and immediately after it is terminated females climb into the canopy to deposit their eggs. Mating success, in the context of this investigation, refers to female attainment of copulation. In 1980, moths were collected in the Ashley Schiff site. Most were taken from 7:00 p.m. until 11:00 p.m. on the evening of 1 December. The first week in December was the mating window for adults of that generation because there were few other periods with temperatures high enough for males to fly. Many females with eggs were found on tree trunks throughout the early winter, and they presumably went unmated. On 1 December unmated and mating moths were collected at the same time. Each mating pair was transferred with soft forceps into a cardboard container. The offspring of most of these females were electrophoretically tested to determine the maternal mode of reproduction. It was possible to determine which females had not mated because they were not climbing upward and were usually in a characteristic posture they presumably assume to release pheromones. Where possible, the nearest unmated female on the same tree as a mating pair and an unmated female from the nearest tree were also collected. This pattern of sampling was employed to look for differences between mated and unmated females on a relatively fine spatial scale. All females were frozen and analyzed by electrophoresis, using the techniques of Mitter et al. (1979) and modifications of Futuyma et al. (1981). In this manner, a four-locus genotype was determined for each individual. H A R S H M A N A N D F U T U Y M A , A N N A L S O F T H E E N T O M O L O G I C A L S O C I E T Y O F A M E R I C A 7 8 (1 9 8 5 )