Post-Mating Selection of Hybrid Toads ( Scaphiopus multiplicatus and Scaphiopus bombifrons )

-High frequencies of F I hybrids and the offspring of backcrosses occur in a hybrid zone between the spadefoot toads Scaphiopus multiplicatus and S. bombifrons in southeastern Arizona. To assess potential post-mating isolating mechanisms between these species, reproductive characteristics, tadpole growth rates, and survivorship in the laboratory of the parental species and hybrids were measured and compared. Hybrid males are sterile; hybrid females, although fertile, produce only about half as many eggs as either parental species and are thus at a selective disadvantage. On a heterogeneous diet including naturally occurring foods, tadpoles of all types develop faster and are larger than those fed a trout-food diet. Hybrid tadpoles have higher survival rate on both diets and develop faster on a mixed diet than does either parental species. At metamorphosis hybrids are intermediate in size between the parental species. These developmental advantages may afford hybrids some compensation for adverse fertility and fecundity selection. genotypes are often present. This suggests that other forces are at work. For example, heterotic effects in the development or Investigations of hybridization and hybrid zones have been survivorship of hybrid offspring, such as have been reported in important in the development of our understanding of the processes other systems (Wasserman, 1957, 1963; Thornton, 1955; Volpe, of evolution (Barton and Hewitt, 1981; Templeton, 1981; Hewitt, 1960; Pasdar et al., 1984; Samollow and Soule, 1983), might buffer 1988). The hybrid systems that are most informative are those for other fitness reductions . which we can evaluate not only the conditions under which inter­ This paper reports on laboratory measurement of several as­ breeding occurs but also the factors affecting the hybrid offspring pects of post-mating selection in Scaphiopus, including adult fer­ produced and the way in which they respond to their environment. tility and fecundity, egg viability, and tadpole survivorship and An area of hybridization between the spadefoot toads developmental rate. Scaphiopus multiplicatus and Scaphiopus bombifrons (subgenus Spea) in the San Simon Valley of southeastern Arizona was inves­ INTRODUCTION MATERIALS AND METHODS tigated over a four-year period (Simovich, 1985, in press). In this area hybridization and introgression allow the frequencies of hybrid Pairs in amplexus and unmated females were collected from tadpoles to vary spatially and temporally from 0 to 40%, forming a ponds and cattle tanks in the San Simon Valley of southeastern dynamic mosaic. A portion of this variability can be accounted for Arizona. The identity of individuals was determined by protein by the observation that mating in large ponds is species-specific, electrophoresis methods described by Simovich and Sassaman but in small (crowded) ponds more heterotypic matings occur and (1986). The toads were identified to one of six classes: S. hybrid tadpoles are more frequent (Simovich, 1985). multiplicatus (M), S. bombifrons (B), F j hybrids (H), backcross-M Significant pre-mating isolating mechanisms in at least the large (BKM), the offspring of a mating between a female hybrid and a ponds might indicate that interbreeding lowers fitness, and that male S. multiplicatus, backcross-B (BKB), the offspring of a mat­ hybrid offspring are at a disadvantage relative to non-hybrid off­ ing between a female hybrid and a male S. bombifrons, or double­ spring. However, frequencies of adult hybrids are as high as 31% in backcross (DBK), the offspring of matings involving these back­ some breeding choruses, and substantial numbers of introgressed crosses. This procedure uses four independent loci and tan cor­ 2 Marie A. Simovich, Clay A. Sassaman, and Amy Chovnick rectly identify pure species, F I hybrids, and 87% of individuals re­ sulting from backcrosses (Simovich and Sassaman, 1986). Fertility and Fecundity. -Amplexed pairs were allowed to breed in buckets in the laboratory. The number of eggs laid was counted, and random samples (averaging 160) were transferred to plastic trays of aerated water. Embryos were raised to the point where they could swim freely (about four days old), at which time fertility and the percent successful hatch was determined. Com­ parisons of fertility and successful hatch were made for the three most frequent crosses (the two intraspecific crosses and the back­ cross of F I hybrid females to S. multiplicatus males). To estimate fecundity in unmated females, egg counts of a weighed subsample of ovary were extrapolated to the total ovarian weight. Comparisons were limited to the three most common classes of females (S. multiplicatus, S. bombifrons, and F1 hybrid). The ovaries from some of the females that had bred were also examined, and since none of these had significant numbers of mature eggs remaining, the numbers of eggs laid by mated females in the fertility experiments were also used to estimate fecundity. Survival and Developmental Rate. -Tadpoles from selected crosses were combined in an experiment designed to determine if the developmental patterns of the classes differed and if diet or density had significant effects on these patterns. For this, mixtures of tadpoles were kept under two density (high and low) and two food (trout food and mixed diet) regimes. Unfortunately, raccoons destroyed the low-density treatment, so only early, qualitative com­ parisons are available. In the high-density treatment, groups of swimming tadpoles (which had hatched on the same day during the fertility experi­ ments) were placed in four 10-liter buckets. Each group consisted of 60 S. multiplicatus (M) (sampled from three clutches), 60 S. bombifrons (B) (also sampled from three clutches), and 59 F( hybrids (H) (from the one clutch that was available). The buckets were kept outdoors in a screened enclosure that received late morning and afternoon sunlight. Water temperatures were taken daily, and buckets were rotated so that similar tempera­ tures could be maintained. Two replicates of each food treatment were used. The first was a trout-food treatment, in which tadpoles were fed ad libitum with commercial high-protein trout chow. The second was a mixed-diet treatment, in which the ad libitum trout diet was supplemented daily with live fairy shrimp (an important component of the natural diet) in excess of demand. Toadlets that had completed metamorphosis were collected and frozen at two-day intervals.Those that died in the process werefrozen as found. The experiment was terminated while some tadpoles still remained, and these were also frozen. All individuals were identified by electrophoresis, and three aspects of performance were deter­ mined: survivorship (the proportion of the initial sample surviving at the end of the experiment), successful metamorphosis (proportion of survivors completing metamorphosis by the end of the experiment), and developmental rate (average number of days to metamorphosis). Size at metamorphosis was also recorded for each individual.