Ecological speciation without host plant specialization; possible origins of a recently described cryptic Papilio species

North American Papilio canadensis and P. glaucus (Lepidoptera: Papilionidae, these Papilio = Pterourus) have previously been described as having allopatric distributions separated by a narrow hybrid zone running from Minnesota to southern New England, and southward in the Appalachian Mountains (possibly to northern Georgia). Recent patterns of hybridization and introgression suggest a more complex interaction between the two, possibly even resulting in the formation of a new species (Pterourus appalachiensis Pavulaan & Wright, 2002). Recently, extensive northward interspecific introgression of P. glaucus‐diagnostic traits has been observed in the hybrid zone. These include wing bands and other color patterns, the ability to feed on tulip tree leaves, and Hk‐100 allozymes; all are autosomally encoded. However, there has been little northward introgression of certain other P. glaucus traits (such as facultative diapause and bivoltinism, and Ldh‐100 allozymes, both X‐linked; and the Y‐linked melanic mimicry gene in females). Interspecific recombination of the X‐chromosome has evidently occurred, as shown by discordant patterns of X‐linked markers. The P. glaucus X‐linked Pgd‐100 and Pgd‐50 alleles have introgressed 200–400 km north of the historical hybrid zone, yet the P. glaucus X‐linked Ldh‐100 allele has not. The allele frequency shift for both genes is more closely related to the ‘thermal landscape’ (i.e., accumulated degree‐days above a developmental base threshold of 50 °F (=10 °C)) than to latitude. Delayed post‐diapause eclosion of cohorts within the hybrid zone, e.g., the New York/Vermont border area, has produced a natural ‘false‐second generation’ flight (a hybrid swarm of synchronous males and females, where 2300–2700 °F degree‐days have accumulated each year since 1998) that is reproductively isolated from flights of both parental species. Moreover, the newly described P. appalachiensis exhibits a unique combination of traits. These include obligate diapause, a univoltine habit, and the Ldh‐80 or Ldh‐40 alleles (as for P. canadensis), the Pgd‐100 or Pgd‐50 alleles (as for P. glaucus), and a delayed ‘false‐second generation’ reproductive flight period (as observed in the hybrid zone). Since 2001, a rare allele or ‘hybrizyme’ (Ldh‐20) has appeared in this false second generation at high frequencies (40–50%). We hypothesize that strong selection against the facultative diapause (od‐)trait (and the linked Ldh‐100 allele) in regions with 2800 °F degree‐days or less, and divergent selection in favor of Pgd‐100 (or a closely linked trait) combined with allochronic reproductive isolation, has resulted in recombinational, parapatric, hybrid speciation. There is no evidence at present that host‐plant shifts or changes in sex pheromones have driven this process, in contrast to many other speciation events in the Lepidoptera.     

Genetic evaluation with autosomal and X-chromosomal inheritance

Summary At present, genetic evaluation in livestock using best linear unbiased prediction (BLUP) assumes autosomal inheritance. There is evidence, however, of X-chromosomal inheritance for some traits of economic importance. BLUP can accommodate models that include X-chromosomal in addition to autosomal inheritance. To obtain BLUP with autosomal and X-chromosomal additive inheritance for a population in which allelic frequency is equal in the sexes, and that is in gametic equilibrium, we write y _i = x_i + a_i + s_i + e_i, where y _i is the phenotypic value for individual i, x_i, is a vector of constants relating y _i to fixed effects, is a vector of fixed effects, a _i is the additive genetic effect for autosomal loci, S _i is the additive genetic effect for X-chromosomal loci, and e _i is random error. The covariance matrix of a _i's is A _A ² , where A is the matrix of twice the co-ancestries between relatives for autosomal loci, and _A ² is the variance of additive genetic effects for autosomal loci. The covariance matrix of s _i's is S _F ² , where S is a matrix of functions of co-ancestries between relatives for X-chromosomal loci and _F ² is the variance of additive genetic effects for X-chromosomal loci for noninbred females. Given the covariance matrices of random effects a _i, s_i, and e _i, BLUPs of autosomal and of X-chromosomal additive effects can be obtained using mixed model equations. Recursive rules to construct S and an efficient algorithm to compute its inverse are given.Dedicated to the memory of Dr. C. R. Henderson, whose encouraging comments stimulated the research in this paper. Supported in part by the Illinois Agricultural Experiment Station, Hatch Project 35-0367, Estimation of Genetic Parameters.     

Covariance between relatives for X-chromosomal loci in a population in disequilibrium

Summary According to Hardy-Weinberg, for a single autosomal locus, a population achieves equilibrium in one generation of random mating if allelic frequency is the same in the sexes, or in two generations if the frequency is not. For a single X-chromosomal locus, however, the approach to equilibrium oscillates and is gradual. Covariances between relatives for autosomal and for X-chromosomal loci are in the literature for a random mating population in equilibrium. Although assumption of equilibrium is defensible for an autosomal locus, it is less defensible for an X-chromosomal locus. Covariances between collateral and between lineal relatives are derived for X-chromosomal loci in a random mating population not in equilibrium. Collateral relatives such as sibs are of the same generation, and lineal relatives such as parent-offspring are of different generations. Coefficient of co-ancestry between relatives, based on identity by descent, was used in this development. Results are applicable to crossbreeding in livestock and poultry, and also to haplo-diploid organisms, such as the honeybee, in which the entire genome is equivalent to being X-chromosomal.Supported in part by the Illinois Agricultural Experiment Station, Hatch Project 35-0367     

Molecular genetics and heterogeneity in manic depression

Recent research has shown that there are X-linked and possibly chromosome 11-linked forms of manic depression as well as at least one other autosomal form. Segregation analyses of large affected families and the finding of genetic linkage between chromosome specific markers and manic depression mutations provide strong evidence that bipolar as well as unipolar forms of manic depression (MD) within the same family are inherited as a dominant gene disorder. This clarification of the etiology of certain types of depression should bring changed attitudes within psychiatry and may serve to stimulate discussion of the role of evolutionary mechanisms. From a clinical point of view, it has now become possible to determine whether clinical (phenotypic) variation reflects the underlying genotypic heterogeneity of linkage. A preliminary analysis of data from four recent studies shows that there is no clear correlation between such clinical features as the ratio of unipolar to bipolar cases and the genotypic form of manic depression. Further recombinant DNA research, proven to be successful in other genetic diseases, can soon be applied to manic depression. The specific problems posed by manic depression for these techniques are discussed.     

Sex-linked expression of a sexually selected trait in the stalk-eyed fly, Cyrtodiopsis dalmanni

Recent theoretical and empirical work has suggested that the X chromosome may play a special role in the evolution of sexually dimorphic traits. We tested this idea by quantifying sex chromosome influence on male relative eyespan, a dramatically sexually selected trait in the stalk-eyed fly, Cyrtodiopsis dalmanni. After 31 generations of artificial sexual selection on eyespan:body length ratio, we reciprocally crossed high- with low-line flies and found no evidence for maternal effects; the relative eyespan of F1 females from high- and low-line dams did not differ. However, F1 male progeny from high-line dams had longer relative eyespan than male progeny from low-line dams, indicating X-linkage. Comparison of progeny from a backcross involving reciprocal F1 males and control line females confirmed X-linked inheritance and indicated no effect of the Y chromosome on relative eyespan. We estimated that the X chromosome accounts for 25% (SE = 6%) of the change in selected lines, using the average difference between reciprocal F1 males divided by the difference between parental males, or 34%, using estimates of the number of effective factors obtained from reciprocal crosses between a high and low line. These estimates exceed the relative size of the X in the diploid genome of a male, 11.9% (SE = 0.3%), as measured from mitotic chromosome lengths. However, they match expectations if X-linked genes in males exhibit dosage compensation by twofold hyperactivation, as has been observed in other flies. Therefore, sex-linked expression of relative eyespan is likely to be commensurate with the size of the X chromosome in this dramatically dimorphic species.     

GENETIC VARIATION IN BABOON CRANIOFACIAL SEXUAL DIMORPHISM

Sexual dimorphism is a widespread phenomenon and contributes greatly to intraspecies variation. Despite a long history of active research, the genetic basis of dimorphism for complex traits remains unknown. Understanding the sex-specific differences in genetic architecture for cranial traits in a highly dimorphic species could identify possible mechanisms through which selection acts to produce dimorphism. Using distances calculated from three-dimensional landmark data from CT scans of 402 baboon skulls from a known genealogy, we estimated genetic variance parameters in both sexes to determine the presence of gene-by-sex (G × S) interactions and X-linked heritability. We hypothesize that traits exhibiting the greatest degree of sexual dimorphism (facial traits in baboons) will demonstrate either stronger G × S interactions or X-linked effects. We found G × S interactions and X-linked effects for a few measures that span the areas connecting the face to the neurocranium but for no traits restricted to the face. This finding suggests that facial traits will have a limited response to selection for further evolution of dimorphism in this population. We discuss the implications of our results with respect to the origins of cranial sexual dimorphism in this baboon sample, and how the genetic architecture of these traits affects their potential for future evolution.     

Polymorphism with migration and selection

Summary Two single-locus, deterministic models with discrete nonoverlapping generations are formulated for the maintenance of genetic variation in each of two distinct biological situations. The first two models are applicable to an autosomal locus in an hermaphroditic plant population with mixed selfing and random mating. They describe the interaction of migration and viability selection for, respectively, an island migration model and for a subdivided population. Pollen as well as seed may disperse. Sufficient conditions are derived and discussed for the existence of protected polymorphism in the diallelic case. The remaining two models are pertinent to migration and selection at a single X-linked locus. An island model is again considered as well as that of a subdivided population. Mating is at random, selection occurs only through viability differences, and the migration structure for males and females may differ. For a diallelic population, protection conditions are derived and discussed vis-à-vis the autosomal case.M.M. was supported by a U.S. Public Health Service training grant (Grant No. GM780).     

Immunogenetic studies of rhesus monkeys. 6. Absence of the human Xga blood group on rhesus erythrocytes1

It was not possible to demonstrate Xg^a on the erythrocytes of any of 140 rhesus monkeys of both sexes tested with human antiserum rendered specific for Xg^a by absorption.