Sexual Progenitors Research Articles

Histocompatibility analysis of clonal diversity in unisexual hybrids of the killifishes <i>Fundulus heteroclitus</i> and <i>Fundulus diaphanus</i>

Hybrids of the killifishes Fundulus diaphanus and Fundulus heteroclitus, found at two sites on the Atlantic coast of Nova Scotia, are unisexual diploid gynogens. Because there seemed to be many opportunities for the sexual progenitor species to hybridize and form new clones at these sites, we initially had expected that clonal diversity might be high among the hybrids. However, examination of their histocompatibility genomes, via inter-individual scale grafting, provides evidence, albeit not conclusive, that most of the hybrids at the two sites, which are separated by 125 km, comprise a single clone. Grafts made among F. diaphanus were rejected within an average of 16-20 days, establishing that sufficient diversity exists within the histocompatibility genomes of these fish to permit their immune systems to reject foreign melanophores. Grafts made from F. diaphanus to the hybrids were also rejected, within an average of 17-26 days, demonstrating that the hybrids possessed competent immune systems. Grafts m...

Niche Relationships of Clonal and Sexual Fish in a Heterogeneous Landscape

We examined spatial, physiological, and morphological niche relationships in a Phoxinus eos–neogaeus gynogenetic complex of cyprinid fish in five drainages influenced by beaver (Castor canadensis) activity along the 294-km2 Kabetogama Peninsula in Voyageurs National Park of northern Minnesota. Assessment of clonal variation using “in gel” DNA fingerprinting of gynogens from three drainages indicated that all drainages contained only a single clone. When environments associated with beaver pond succession were pooled across drainages, P. eos was more abundant in active beaver ponds, while gynogens were at higher frequencies in collapsed pond and stream environments. Gynogen frequency for different successional environments classified according to drainage indicated strong variation among drainages, especially in pond environments, but a consistent pattern of increased gynogen frequency in collapsed ponds or streams. Intensive sampling within one drainage across successional beaver pond sites, physical habitats, and gradients of oxygen concentration indicated that P. eos, P. neogaeus, and gynogens were all most abundant in the highly oxygenated littoral environment of an upland pond. P. eos abundance and its relative frequency in the gynogenetic complex decreased, while gynogen frequency increased, in a poorly oxygenated collapsed beaver pond and beaver meadow. Fish transplant experiments revealed that gynogens survived longer and had greater physiological tolerance to oxygen stress than sexual progenitors, especially P. eos. Principal component analysis of 11 growth-related morphometric characteristics revealed morphological intermediacy of the hybrid gynogens between the sexual progenitors, with factors related to trophic morphology, especially mouth width and mouth length, segregating the different forms. A comparison of morphological relationships in a beaver pond with a gynogen frequency of 25–35% to those of a pond with a gynogen frequency of only 6–7% suggested that there was a release in the trophic morphology of P. eos in response to reduced gynogen abundance. The temporal and spatial unpredictability of environmental conditions in the north temperate landscape inhabited by these fish has apparently resulted in selection for a physiologically flexible, “general purpose” clonal genotype, which occupies a broad ecological niche and may make establishment of additional clones in the same ecosystem difficult.

REDUCED MALE ALLOCATION IN THE PARTHENOGENETIC HERMAPHRODITE DUGESIA POLYCHROA.

Parthenogenetic lineages that arise in a hermaphroditic, sexual population will inherit the male function from their sexual progenitors. Natural selection then acts to reduce male allocation of the parthenogens, freeing resources presumably for the female function. Depending on age and the available genetic variation, one therefore expects to find reduced male allocation in naturally occurring parthenogenetic lineages. We investigated the allocation to sperm production in the hermaphroditic flatworm Dugesia polychroa in three lakes containing a sexual (S), a (pseudogamous) parthenogenetic (P), and a mixed sexual-parthenogenetic population (M). Parthenogenetic lineages from M were assumed to be relatively young due to recurrent origins from the coexisting sexuals, whereas those from P were assumed to be older on biogeographical grounds. As predicted, we found drastically reduced sperm production in parthenogens compared to sexuals, even in the parthenogenetic lineages from M, which may be younger. M parthenogens did not have more testes, but produced more sperm than individuals from the purely parthenogenetic population (P). However, the latter result could not be reproduced with laboratory-raised animals and therefore may be a consequence of different ecological conditions in the different lakes, for example, differences in mating rates. To study the behavioral component of male allocation, copulation frequencies were recorded for sexuals from M and for parthenogens from P. Compared to the drastic reduction in sperm production, copulation frequency was less reduced in parthenogens. This may be a consequence of allosperm limitation in pseudogamous parthenogenetic populations.

Genome Size of Bisexual and Unisexual Poeciliopsis

We used flow cytometry to examine the genome size of bisexual and unisexual fishes in the genus Poeciliopsis. Two bisexual species, P. monacha and P. lucida, had genome sizes of 1.45–1.47 pg DNA and 1.37 pg DNA, respectively. These are small genomes for a vertebrate. Hemiclones of the unisexual hybrid P. mona-cha-lucida had genome sizes intermediate to those of P. monacha and P. lucida, its sexual progenitors. The triploid unisexuals P. 2 monacha-lucida and P. monacha-2 lucida had genome sizes equal to the sum of their three haploid components. No ploidy mosaics were observed among the unisexuals. Their genome size has not changed over time from that of the parental species that hybridized and produced them.

Comparisons of life-history traits between clonal and sexual fish (Poeciliopsis:Poeciliidae) raised in monoculture and mixed treatments

Clonal and sexual co-existence is common in a number of vertebrate taxa, even though the ‘cost of sex’ makes such co-existence theoretically unlikely. The frozen niche-variation (FNV) model explains this co-existence on the basis of differences in overall niche breadth and competition between clones and sexuals. In the present study I examined two predictions of the FNV model. First, I examined the prediction that genetically variable populations have higher relative fitness when compared with monoclonal populations by comparing the performances of clonal and outcrossed sexual strains ofPoeciliopsis in monocultures at two densities. The prediction of increased overall productivity for the sexuals was verified, with net reproductive rates for the sexuals being between two and four times as high as the clones. Second, I tested the prediction that derived clones will successfully compete with their sexual progenitor(s) in the narrow range to which the clones are adapted, while the sexuals should co-exist because of their ability to use a wider range of resources than any single clone. I examined this prediction by comparing performance variables (e.g. growth, fecundity and survival) of each strain in pure culture with their partitioned performance from the mixed treatments. Clonal performance increased in mixtures compared to monocultures, as expected. However, the expectation that the sexual's performance would be less affected by mixtures than the clones' performance, was not met. The sexuals had reduced growth and fecundity on a par with the increase in both variables in the clones. Therefore, support for the FNV model was mixed. Although the performance in monocultures suggests that the sexuals have a wider niche breadth than the clones, performances in mixtures do not indicate such a relationship. Switching of behaviours or resource-use patterns between mixed and pure cultures may have caused the equivocal results.

Feeding behavior in sexual and clonal strains of Poeciliopsis

Sexual and clonal fish of the genus Poeciliopsis occur together in desert streams of Sonora, Mexico. Their coexistence has been explained in terms of niche partitioning for food and space. We examined predatory behavior that might influence niche relationships, and found significant differences among two coexisting sperm-dependent clonal strains and their two sexual progenitors. Handling time and prey manipulation of free-swimming (Artemia) and benthic (chironomid larvae) prey differed significantly among sexual and clonal strains. Analyses of gut contents from field-collected fish revealed that the laboratory estimates of predatory efficiency were related to their feeding behavior in nature. Behavior differences, such as those described herein, contribute to our understanding of the mechanisms of unisexual/bisexual coexistence in Poeciliopsis.

Patterns of Clonal Diversity in Geographically Marginal Populations of Antennaria rosea (Asteraceae: Inuleae) from Subarctic Alaska and Yukon Territory

Antennaria rosea (Asteraceae: Inuleae) is a herbaceous perennial that reproduces by gametophytic apomixis and is widespread in the cordillera of western North America, ranging from New Mexico to Alaska. In the overall pattern of population structure in A. rosea, the majority of the populations are polyclonal, although the average number of clones per population is relatively small (X̄ = 3.1). In general, clones are restricted to one or two populations (X̄ = 1.1) and, therefore, very few widespread clones exist. The subarctic populations are characterized by relatively low amounts of clonal diversity when compared with those from southern latitudes (X̄ = 2.0/population). Clonal diversity is negatively correlated with latitude, longitude, and elevation of the sites. It seems unlikely that A. rosea survived the last glacial episode in the northern glacial refugium, Beringia, and a more likely scenario for the phytogeographic pattern of clonal diversity is one that envisions A. rosea surviving the Wisconsinan south of the glacial margin or perhaps arising in the same region at the end of the Wisconsinan. New clones probably arise via crossing of facultatively apomictic clones of A. rosea to their sexual progenitors. As most of the eight sexual progenitor taxa are absent from the north, new clones cannot easily arise there. The lack of clonal diversity in the subarctic and arctic could be the result of a combination of recent migration and lack of sexual progenitors in the region.

Investigations into the evolutionary history of theAntennaria rosea (Asteraceae: Inuleae) polyploid complex

TheAntennaria rosea polyploid agamic complex is one of the most morphologically diverse and widespread complexes of N. AmericanAntennaria. The group is taxonomically confusing because of numerous agamospermous microspecies, having been recognized as distinct species. Morphometric analyses have demonstrated that the primary source of morphological variability in the complex derives from six sexually reproducing progenitors,A. aromatica, A. corymbosa, A. media, A. microphylla, A. racemona, andA. umbrinella. Additionally, two other sexually reproducing species,A. marginata andA. rosulata, may have contributed to the genetic complexity of theA. rosea complex. Cluster analysis indicates that four discrete morphological groups exist within theA. rosea complex. Each group could be the result of predominance of genes from different groups of sexual progenitors. AsA. rosea is of multiple hybrid origin, from among several sexual progenitors, it is advisable to recognizeA. rosea as a distinct species from its sexual progenitors.

The Influence of Climatic Factors on Clonal Diversity in a Population of Iris pumila

An attempt is made to determine whether clonal colour diversity currently existing in a population of Iris pumila was causally related to variation in some climatic factors. It was observed that temporal changes in temperature condition affect the magnitude of clonal diversity in the I. pumila population. A highly significant positive correlation was found between the cohort diversity and the mean spring temperatures at the flowering phase of their sexual progenitors, and a significant negative correlation between cohort diversity and the mean winter temperatures two years before the cohorts were detected. Differential requirements of various life-stages of I. pumila for particular temperature conditions, as well as differential pollen transfer rate at different temperatures might explain these inverse correlations. It is supposed that a combination of fluctuating temperatures giving rise to environmental heterogeneity and a varied attractiveness of diverse colour morphs to pollinators promotes stable coexistence of multiple colour variants in the population of I. pumila.

Patterns of Isozyme Variation in the Antennaria rosea (Asteraceae: Inuleae) Polyploid Agamic Complex

Antennaria is a genus of dioecious, perennial herbs, which are distributed over much of North America and Eurasia. The A. rosea polyploid agamic complex occurs throughout western North America and is perhaps the most complicated, with respect to morphology and taxonomy, of all the Antennaria species complexes. The complex is composed of gametophytic apomicts and consequently the populations contain only pistillate clones. Morphological studies have demon- strated that the A. rosea complex is a compilospecies derived through hybridization from perhaps as many as eight, sexually reproducing species of Antennaria, with which it has a partially sympatric distribution. Enzyme electrophoresis was used both to assess the degree of divergence among 33 populations of A. rosea, as well as to determine the amount of divergence between A. rosea and its sexual progenitors. Gene statistics demonstrate that A. rosea contains more gene diversity than its amphimictic relatives. The electrophoretic data provide evidence, independent, yet supportive, of morphology, that A. corymbosa, A. microphylla, and A. umbrinella are among the sexual progenitors of some of the A. rosea segregates. Morphology also implicates A. aromatica, A. densifolia, A. marginata, A. media, A. racemosa, and A. rosulata in the parentage of A. rosea, although the electrophoretic data neither support nor contradict this hypothesis independent of morphology. The A. rosea complex is probably of recent origin because it has diverged only moderately from its sexual progenitors, having only one novel allele that does not occur in the amphimicts.

TYPIFICATION OF WESTERN NORTH AMERICAN ANTENNARIA GAERTNER (ASTERACEAE: INULEAE); SEXUAL SPECIES OF SECTIONS ALPINAE, DIOICAE, AND PLANTAGINIFOLIAE

SummaryIt has been shown that eight well‐defined species of Antennaria should be recognized (A. aromatica, A. corymbosa, A. marginata, A. media, A. microphylla, A. racemosa, A. rosulata, and A. umbrinella). A proposed synonymy for these eight diploid species, based on the similarity matrix and discriminant analysis, is presented here. These species appear to be the sexual progenitors for the polyploid agamic complexes comprising A. rosea and A. parvifolia. Relationships between the diploids and the polyploids are discussed.

Geographic Dispersal and Clonal Diversity in Unisexual Fish Populations

Tissue grafting was used to investigate clonal diversity in populations of the unisexual hybrid fish, Poeciliopsis monacha-occidentalis from five rivers in northwest Mexico. The ranges of the two sexual progenitor species, P. monacha and P. occidentalis, overlap only in the southernmost of these rivers. Synthesis of new clones of P. monacha-occidentalis via interspecific matings may still occur there. The other four rivers are north of the range of P. monacha, but contain populations of P. monacha-occidentalis and its sexual host species, P. occidentalis. A surprisingly large number of clones was detected in the southernmost river. Unisexual strains descended from 33 wild-collected females comprised 17 distinct clones. Clonal diversity was drastically lower in the northern rivers. In one of the rivers two clones were detected; the other three rivers had only one clone each. This pattern strongly indicates that multiple-hybrid origins are the most important source of clonal diversity in P. monacha-occidentalis. Evolution does not cease with the advent of unisexuality, however. An example of genetic change in unisexuals subsequent to their hybrid origin was detected. The single clone from the northernmost river contains unique alleles at two electrophoretic loci. This clone was found to be antigenically similar to the single clone from the adjacent river. It is likely that these two clones have diverged genetically from a common ancestral clone that originated in the south. Factors affecting the clonal composition of populations are discussed. The clonal composition of the populations is not a result of random processes. Among the selective forces influencing clonal diversity, habitat complexity probably plays a minor role in comparison to the high degree of temporal instability which is characteristic of these desert stream habitats.