Clay Sassaman

The genetic mechanism of sex determination in the conchostracan shrimp Eulimnadia texana.

We report the results of a laboratory pedigree analysis describing the unique sex-determining mechanism of the conchostracan shrimp, Eulimnadia texana. Natural populations of E. texana are mixtures of self-compatible hermaphrodites and males and represent one of the few known cases of androdioecy in animals. Hermaphrodites are of two types: amphigenic (producing both male and hermaphroditic offspring) and monogenic (producing only hermaphroditic offspring). We propose a simple genetic model to explain this polymorphism and show by genetic analysis that males, amphigenics, and monogenics can be interpreted as three alternative phenotypes of a one-locus system of sex determination. We discuss the implications of this novel system of sex determination for understanding the evolution of reproductive systems.

Rocky intertidal fish communities of California: temporal and spatial variation

SynopsisWe examined data from our own and published collections of intertidal and shallow littoral fishes of the North American Pacific Coast with respect to temporal and spatial trends in species composition and dominance. We compared (1) recent and past intertidal collections made five and seven years apart, respectively, for two California localities, (2) intertidal collections from twelve localities in California, Oregon, and British Columbia, and (3) intertidal versus subtidal collections at one California locality. Temporal comparisons indicated substantially lower abundance of the cottid Oligocottus snyderi at two California localities during 1984, at one locality due to depressed abundance of young. Interannual differences in abundances suggested that other tidepool fish assemblages undergo significant changes as well. Geographical comparisons indicated general similarity in species composition, with cottids predominating in tidepools although several other families also were well represented (e.g., Stichaeidae, Scorpaenidae). Eel-shaped stichaeids and pholids occurred at high densities in exposed boulder fields. Both the tidepool and boulder field assemblages showed north-south changes in species abundances. Comparison of collections from the intertidal and subtidal zones at one California locality demonstrated that fishes of these habitats form two essentially distinct assemblages, with most species restricted to or concentrated in one or the other habitat.

Genetic interaction between male mating strategy and sex ratio in a marine isopod

Individual males in many animal species exhibit discrete modes of behaviour, but the genetic mechanisms underlying these differences are poorly understood. Here we investigate the genetics of the isopod crustacean Paracerceis sculpta, in which three different types of males coexist, each distinguishable from the others by their behavioural and morphological phenotypes. Within families, alleles of the gene encoding the enzyme phosphoglucomutase (Pgm gene) are associated with particular male phenotypes, although no significant association between these characters exists population-wide. This suggests that Pgm is closely linked to a single genetic locus which controls male phenotype. We call this the alternative mating strategy (Ams) locus. We present evidence that two other factors—an autosomal gene, transformer (Tfr), and an extrachromosomal factor—interact with primary sex determination loci and with alleles at Ams, causing certain individuals to change sex, thereby biasing family sex ratios. A model based on our genetic analysis suggests that: first, polymorphism in male behaviour is controlled by the mendelian segregation of three alleles at the Ams locus; second, that family sex ratio is influenced by alternative alleles at the Tfr locus whose expression is influenced by the extrachromosomal factor; and third, that Tfr and Ams interact epistatically to determine the sex of the individual and, if male, its behaviour and external morphology.

PORCELLIONIDES FLORIA, NEW SPECIES, FROM NORTH AMERICA; PROVINCIALITY IN THE COSMOPOLITAN ISOPOD PORCELLIONIDES PRUINOSUS (BRANDT)

ABSTRACT Porcellionidespruinosus (Brandt) is widely distributed throughout the world but shows extensive, albeit subtle, geographical variation in morphology. This morphological variation has resulted in a complex and confused taxonomic history for the genus, with many new species having been described only to be subsequently synonymized with P. pruinosus. What has previously been considered to be P. pruinosus in North America is shown here to be two separate species. Although these species differ only subtly in morphology, they are electrophoretically, reproductively, and distributionally distinct. For one of these species, P. pruinosus(Brandt), electrophoretic, morphological, and reproductive comparisons of North American individuals with European samples suggest a European origin. The other species is not referable to any other described species of Porcellionides and is described as a new species. The results of this study suggest that Porcellionides pruinosus (as formerly defined) may consist of a number of distinct and localized species rather than one cosmopolitan species. Therefore, it would appear that the taxonomic status of Porcellionides pruinosus is in need of re-evaluation on a world-wide basis.

Gynandromorphism in Anostraca: multiple mechanisms of origin?

Gynandromorphism is relatively rare in anostracans and is generally thought to arise from mitotic aberrations occurring during early embryogenesis that create somatic-cell mosaics. We have reared 35gynandromorphic Branchinecta, most of them being B. lindahli. The morphology of these individuals, the relatively high frequency of their occurrence, and their aggregation within sibships but not within pedigrees, indicate that they are not the result of mutational events. Their differential appearance in female-biased sibships suggests, instead, that the majority of them represent males that have been incompletely transformed into females by epigenetic feminizing factors.

Competition in phenotypically variable and uniform populations of the tadpole shrimp triops longicaudatus notostraca triopsidae

SummaryLife-history parameters (growth, reproduction, and survival) were measured from one genetically heterogeneous and three homogeneous populations of tadpole shrimp (Triops longicaudatus) raised under three competitive levels: 5, 10, and 16 shrimp per 38-1 aquarium. Comparisons were made between the homogeneous populations (three monomorphic selfing lines) and a heterogeneous population (a mixture of the three lines) to test for increased productivity in genetically variable populations, a prediction of the tangled bank hypothesis for a short-term advantage to sexual reproduction. In most comparisons, the mixture of inbred lines performed better than expected from their individual performances in pure cultures but did not outperform the best performing line in any comparison. Decomposition of the mixed cultures performance into performances of the component inbred lines showed that not all lines experienced release from competition in the mixed culture. No genotype X environment interaction was found for any of the three fitness correlates, indicating that all three lines performed similarly under the three competitive conditions. These results provide little support for the tangled bank hypothesis and suggest that the levels of heterogeneity necessary for detectable resource partitioning in these shrimp may involve factors not considered in this study.

Geographical patterns of allozymic variation in three species of intertidal sculpins

SynopsisWe examined spatial patterns of allozyme variation in three species of intertidal cottids to evaluate how much gene flow occurs between geographically separate populations (most likely via passive dispersal of planktonic larvae by currents). Our results from the analysis of geographical patterns of allele frequencies and, to some extent, from comparison of allele frequencies between newly recruited young and older resident fish are consistent with the notion that sufficient gene flow occurs in these fishes to prevent significant genetic isolation of populations. From these results, we visualize evolutionary changes in populations of these species as occurring most likely over a broad geographic scale, affecting spatially separated but genetically linked populations concurrently, rather than operating independently in individual, genetically isolated populations.

Paradoxical acclimation and seasonal comparisons of oxygen consumption rates in three species ofPogonomyrmex harvester ants (Hymenoptera: Formicidae)

SummarySeasonal comparisons of oxygen consumption rates of three species ofPogonomyrmex harvester ants demonstrate paradoxical acclimation in the laboratory and negative acclimatization in the field.It is usually assumed that paradoxical acclimation reduces energy expenditures during unfavorable seasons. There is no evidence that this occurs in the harvester ants. The ants are able to be active only during a portion of the year, a time in which they raise new workers and reproductives. Laboratory acclimation and field acclimatization studies suggest that ants of all three species increase their metabolism during this time. The increase in above ground activity in summer may result in greater efficiency of the nest in producing new workers and reproductive females and males. Inverse acclimation in these ants does not decrease energy expenditures during unfavorable seasons.In most investigations of ants, the nest is implicitly assumed to be a homogeneous mixture of individuals. One unexpected result of a comparison of nest levels is that nests are stratified in terms of important physiological parameters. Stratification within the nest implies there is little vertical mixing and as a result individuals from different levels were analyzed separately.

TEMPORAL STABILITY OF LACTATE DEHYDROGENASE-A CLINES OF THE HIGH COCKSCOMB, ANOPLARCHUS PURPURESCENS

In marine species with planktonic larval stages, strong geographic differentiation among resident adult populations is noteworthy in that identification of the forces and mechanisms which create such local differentiation allows an assessment of the relative importance of natural selection in promoting genetic divergence. Williams et al. (1973), and later Koehn and Williams (1978), focused on this problem in their analysis of genetic differentiation in the American eel, Anguilla rostrata. Their study centered on the interaction between age-dependent differentiation (elvers vs. resident adults) and spatial differentiation of groups along a latitudinal transect. They found different patterns of differentiation which depended upon the specific genetic marker under analysis. At the Mdh locus, allele frequencies were homogeneous among age-groups and among localities, suggesting recruitment of elvers from a common zygotic pool with no subsequent divergence between groups of colonists in situ. In contrast, at the Sdh locus, they observed a significant spatial component of variation across latitude, but one which was equally pronounced in both age groups. This result suggested (in light of the life history of the American eel) that divergence occurred prior to larval recruitment and was not further accentuated in situ. Finally, at the Phi-II locus, they observed that allele frequencies of newly recruited larvae were uniform across the transect (once again suggesting a common zygotic pool), but that geographic differentiation of adults was significant. This latter pattern would be consistent with a predominant role of local selective factors in promoting genetic divergence and it suggested selection intensities of about 10% per generation. Although the power of their analysis lies largely in the peculiar migrational history of the American eel, there is no reason to believe that the differences in genetic composition between life stages observed by Williams et al. (1973) and Koehn and Williams (1978) are in any way unique to Anguilla. As they point out, it is reasonable to expect that age-specific selection would operate in other species with planktonic larvae, but similar analyses have been rarely conducted. Actually, the degree of geographic differentiation in the American eel is rather slight; for example, systematic variation in Phi-II allele frequencies is on the order of 5% per degree of latitude (Koehn and Williams, 1978). In other marine fishes with planktonic larvae, more pronounced examples of geographic variation are known. In Anoplarchus purpurescens, an intertidal blennioid fish of the northwestern Pacific Ocean, relatively sharp differences in allele frequency over short distances have been reported (Johnson, 1971, 1977; Sassaman and Yoshiyama, 1979; Yoshiyama and Sassaman, unpubl.). Here we explore temporal (age-group) aspects of allele-frequency variation along two pronounced gradients of gene-frequency change in Anoplarchus. Our analysis follows the same logical format of Williams et al. (1973) and Koehn and Williams (1978). In addition we examine yearto-year stability of these gene-frequency clines by reference to previously published information (Johnson, 1971; Sassaman and Yoshiyama, 1979) for the same sampling sites. Since the presumption of extensive

The interaction between the terrestrial isopod Porcellio scaber Latreille and one of its dipteran parasites, Melanophora roralis (L.) (Rhinophoridae)

Melanophora roralis (L.), a rhinophorid fly, was reared from naturally infected sowbugs, Porcellionides pruinosus (Brandt) and Porcellio scaber Latreille, collected at several locations in the eastern United States, the majority from P. scaber. Aspects of the host-parasite interaction were studied in a large sample ofisopods from North Carolina. The flies were sexually dimorphic for body size, and varied in size as a function of individual host size. Differences between the fly sexes were due to their differential utilization of a common range of host sizes rather than to the use of different host sizes; this result was directly confirmed with laboratory rearings of full sibships. Two potential indicators of individual fitness, adult longevity, and female fecundity, were direct functions of parasite, and hence host, size. The size dimorphism in Melanophora roralis, and physiological features associated with it, may reflect adaptations in this parasite for population persistence at low population densities. The Rhinophoridae are dipteran flies largely, or exclusively, specialized as larval parasites of terrestrial isopod crustaceans (Crosskey, 1977). The family is small, with about 100 species currently recognized, and has had a long and confused history of affiliation with both the Calliphoridae and the Tachinidae (for review see Crosskey, 1977; Kugler, 1978). Despite the generalizations often made about rhinophorid natural history, relatively little is known.about these flies. Host records for most species are lacking, and very little information has been published on host-parasite interactions since Thompsons (1934) monograph on the British species. Only one rhinophorid, Melanophora roralis (L.), is widespread in North America. The species ranges from Michigan to Kansas and from New Hampshire to Florida and Louisiana (Sabrosky and Araud, 1965). Although this fly has been frequently collected, it is rarely very abundant (Brues, 1903). M. roralis is known to be a parasite of Porcellio scaber Latreille (Thompson, 1934) and Oniscus asellus L. (Jones, 1948), but details of the host-parasite interaction have received little attention due to the sporadic and rare occurrence of adequate samples. We have occasionally reared M. roralis from isopods collected for other experimental purposes. One of these collections, from North Carolina, had an incidence of parasitism high enough to prompt the present study.