THE EFFECT OF INFLORESCENCE SIZE ON MALE FITNESS: EXPERIMENTAL TESTS IN THE ANDROMONOECIOUS LILY,ZIGADENUS PANICULATUS

We studied the relationship between inflorescence size and male fitness in the andromonoecious lily Zigadenus paniculatus, using experimentally manipulated inflorescences to eliminate possible correlations between flower number, resource availability, and other floral traits. Allozyme markers were used to determine the siring success of large versus small plants in 14 arrays of plants, each array containing five large and five small plants. The inflorescence size of small plants was held constant both within and among arrays; the size of large plants was held constant within an array but was varied among arrays. Large plants sired more than half the seeds in 12 of the 14 arrays, and significantly more than half in six of these 12. However, in eight of the arrays, large plants sired significantly fewer seeds than expected on the basis of their size advantage. Furthermore, there was no significant relationship between relative size and relative siring success in comparisons among arrays. A maximum-likelihood model estimated that 28% of seeds were sired by imported pollen, with 95% confidence limits of 13% and 50%. Within these limits, high import rates tended to mask the relative success of large plants in several arrays. These results suggest that the evolution of inflorescence size in Z. paniculatus is at least partly driven by selection for increased male success, assuming genetic variation for flower number. However, the data also support a growing body of evidence that estimates of male fitness in plants can be highly variable. We discuss the sources of this variability and the possible effects of inflorescence design on the relationship between inflorescence size and fitness.     

A TEST OF THE SPONTANEOUS HETEROSIS HYPOTHESIS FOR UNISEXUAL VERTEBRATES

The coupling between clonal modes of reproduction and hybridization in unisexual vertebrates has led to the hypothesis that heterosis accounts for their ecological success (the “spontaneous heterosis” hypothesis). High levels of genic heterozygosity characteristic of unisexual-hybrid vertebrates are believed to result in enhanced growth, survivorship, and fertility relative to their sexual ancestors. To test this hypothesis, we synthesized 33 new unisexual-hybrid strains of fishes in the genus Poeciliopsis (Atheriniformes: Poeciliidae). On average, the synthetic unisexuals had lower survivorship and a higher incidence of birth defects than either of the sexual ancestors or two natural strains of unisexuals. However, a subset of these synthetic unisexuals exhibited characteristics within the range of the sexual ancestors and natural unisexual strains. These results support the alternative hypothesis that the ecological success of natural unisexuals results from selection of the most fit clones from a broad spectrum of genotypes that arose via multiple hybrid events. We propose that the coupling between unisexuality and hybridization in the vertebrates exists because hybridization is a dysgenic process that can disrupt normal gametogenesis and thus lead to clonal reproduction.     

INTRASPECIFIC PHYLOGEOGRAPHY OF NORTH AMERICAN HIGHLAND FISHES: A TEST OF THE PLEISTOCENE VICARIANCE HYPOTHESIS

The highland fish fauna of eastern North America consists of Appalachian and Ozark centers of endemism separated by the intervening Glacial Till Plains. Clades within these areas are more closely related phylogenetically to each other than to clades occurring in the intervening formerly glaciated region, suggesting that the Pleistocene glaciations fragmented a widespread highland region and its associated fauna. Alternatively, it is possible that these faunal assemblages predate the glaciations or that recent dispersals may have been more important than vicariance in determining faunal compositions. We examined the relationships among mitochondrial DNA (mtDNA) haplotypes within five clades of highland fishes, each with a distribution suggestive of a Pleistocene vicariance event. Darters of the subgenera Litocara and Odontopholis have distributions and mtDNA relationships that are consistent with the Pleistocene integration and burial of the Teays-Mahomet valley, a major drainage of the early Pleistocene. The distribution and mtDNA relationships among subspecies of Erimystax dissimilis are not consistent with Pleistocene vicariance, but relationships among Appalachian haplotypes are consistent with the late Pleistocene integration of the modern Ohio River system. Both Cottus carolinae and the Fundulus catenatus species group have representatives in the Mobile basin consistent with pre-Pleistocene divergences. Three haplotype clusters were found in C. carolinae, corresponding to the Appalachian, Ozark, and upper Kanawha River populations. However, Appalachian and Ozark F. catenatus populations are paraphyletic with respect to each other. This, coupled with a relatively low degree of sequence divergence, suggests that no long-term barriers to gene flow exist for C. carolinae and F. catenatus. These three distinct phylogeographic patterns indicate that Pleistocene vicariance is not the only explanation for the Appalachian-Ozark distribution of highland fish communities.     

A COMPARATIVE ANALYSIS OF SEX CHANGE IN LABRIDAE SUPPORTS THE SIZE ADVANTAGE HYPOTHESIS

The size advantage hypothesis (SAH) predicts that the rate of increase in male and female fitness with size (the size advantage) drives the evolution of sequential hermaphroditism or sex change. Despite qualitative agreement between empirical patterns and SAH, only one comparative study tested SAH quantitatively. Here, we perform the first comparative analysis of sex change in Labridae, a group of hermaphroditic and dioecious (non–sex changer) fish with several model sex‐changing species. We also estimate, for the first time, rates of evolutionary transitions between sex change and dioecy. Our analyses support SAH and indicate that the evolution of hermaphroditism is correlated to the size advantage. Furthermore, we find that transitions from sex change to dioecy are less likely under stronger size advantage. We cannot determine, however, how the size advantage affects transitions from dioecy to sex change. Finally, contrary to what is generally expected, we find that transitions from dioecy to sex change are more likely than transitions from sex change to dioecy. The similarity of sexual differentiation in hermaphroditic and dioecious labrids might underlie this pattern. We suggest that elucidating the developmental basis of sex change is critical to predict and explain patterns of the evolutionary history of sequential hermaphroditism.     

A TEST OF THE CHITTY HYPOTHESIS: INHERITANCE OF LIFE-HISTORY TRAITS IN MEADOW VOLES MICROTUS PENNSYLVANICUS

The Chitty hypothesis proposes that the demographic changes occurring in microtine cycles are mediated by natural selection operating on the genetic composition of the population. Implicit in this hypothesis is the assumption that a suite of life-history traits is simultaneously undergoing selection and that these traits are strongly heritable. We tested this in two ways: first, by determining whether the year-to-year differences in phenotypes in fluctuating meadow vole populations in the field are maintained in samples of young animals raised in the laboratory, and second, whether the variation seen in the field has a heritable basis as determined by half-sib analysis. Parents were obtained in the springs of successive years from a fluctuating meadow vole population. These animals were bred in small field enclosures, and their progeny were raised in the laboratory. Animals raised in the laboratory differed significantly from those in the natural field population. In the field, young from the year when population size was increasing grew more rapidly than those from the peak year; in the laboratory, the opposite occurred. The ages at sexual maturity showed similar differences. Heritability analysis was performed on body weight, growth rate, and age and weight at sexual maturity. Virtually all these traits showed significant dam effects, but small or nonexistant sire effects. Thus, most of the variation was nongenetic in origin; maternal and other environmental effects were of overriding importance. We conclude that the heritabilities of these traits in nature are usually lower than necessary for natural selection to operate in the time frames characteristic of microtine cycles.     

A TEST OF THE SIB-COMPETITION HYPOTHESIS FOR OUTCROSSING ADVANTAGE IN IMPATIENS CAPENSIS

One of the potential selective mechanisms invoked in discussions of breeding-system evolution is that competition within sibships increases the fitness of outcrossed progeny relative to selfed progeny. We tested this sib-competition hypothesis using cleistogamous (CL) and chasmogamous (CH) seeds of Impatiens capensis in a large greenhouse experiment. The experimental design was a double replacement series which also allowed us to test for inbreeding depression and overall resource partitioning among sibships. We found no evidence for strong inbreeding depression in the study population; although plants from CH seeds had a slight advantage over plants from CL seeds in total flower and pod production, CL plants had slightly higher growth. We also could not detect significant resource partitioning among sibships nor any evidence to support the sib-competition hypothesis for outcrossing advantage. CH sibships were not significantly more variable than CL sibships in any of the phenotypic traits measured. These results suggest that sibling competition may have little importance in the evolution of Impatiens breeding systems.     

REPRODUCTIVE SUCCESS AND INFLORESCENCE SIZE OF CALOPOGON TUBEROSUS (ORCHIDACEAE)

Reproductive success of Calopogon tuberosus, which produces no nectar, was investigated in relation to inflorescence size and dispersion pattern. Mean inflorescence size was 2.56 (range 1–10). A bagging experiment showed that insects are required for pollen transfer and that fruits are produced from self-, geitonogamous, and cross-pollinations; fruit set was not 100%. Fruit set of nonmanipulated plants was limited by the number of pollinator visits. Reproductive success increased with increasing inflorescence size, although not above theoretical predictions. However, the probability of producing no fruit or contributing no pollinia decreased with increasing inflorescence size since sequential flowering increased the probability of a pollinator visit to the inflorescence over the blooming period. Large inflorescences did not provide a greater pollinator attraction than small ones, because inflorescences only presented a few open flowers at a time. In addition, flowers on plants growing in clumps of 2–8 plants had a higher probability of setting fruit, apparently because of increased pollinator attraction. Although there are obvious selective advantages for large inflorescences, the sequential flowering habit, and low resource availability may reduce the advantages of large inflorescence size at our study site.     

EXPERIMENTAL STUDIES OF THE EVOLUTIONARY SIGNIFICANCE OF SEXUAL REPRODUCTION II. A TEST OF THE DENSITY-DEPENDENT SELECTION HYPOTHESIS

This study tests the hypothesis that one evolutionary advantage of sexual reproduction is that it produces genetically variable progeny with a density-dependent advantage mediated by resource partitioning or pest pressure. Our experimental approach involved planting separate plots of sexually-derived and asexually-derived tillers of the grass Anthoxanthum odoratum in density gradients at the two natural sites from which the source material was taken. The sexual progeny displayed a significant fitness advantage compared to the asexual progeny. But, in contrast to the expectations of the density-dependent selection hypothesis, the advantage of the sexually produced progeny is most marked at lower densities. Thus, the results of this experiment and our previous report (Antonovics and Ellstrand, 1984) seem to best support the frequency-dependent selection hypothesis for the advantage of sexual reproduction.     

步氏巨猿牙齿大小上的变异性和南方古猿类食性假说

步氏巨猿牙齿大小上的变异性表明,在柳城巨猿洞局部地区的堆积中可能有少量时代稍晚的巨猿牙齿标本。晚期步氏巨猿与早期的相比,后部齿显著增大而前部齿则无显著差异。从步氏巨猿牙齿在大小上的演化趋势来看,南非南方古猿类中的纤细类与粗壮类之间在齿列比例上的不同不一定意味着其食性上有大的差异,纤细类与粗壮类也未必有“属”这一分类级别上的差异。     

EVOLUTION OF VIVIPARITY: A PHYLOGENETIC TEST OF THE COLD‐CLIMATE HYPOTHESIS IN PHRYNOSOMATID LIZARDS

The evolution of viviparity is a key life‐history transition in vertebrates, but the selective forces favoring its evolution are not fully understood. With >100 origins of viviparity, squamate reptiles (lizards and snakes) are ideal for addressing this issue. Some evidence from field and laboratory studies supports the “cold‐climate” hypothesis, wherein viviparity provides an advantage in cold environments by allowing mothers to maintain higher temperatures for developing embryos. Surprisingly, the cold‐climate hypothesis has not been tested using both climatic data and phylogenetic comparative methods. Here, we investigate the evolution of viviparity in the lizard family Phrynosomatidae using GIS‐based environmental data, an extensive phylogeny (117 species), and recently developed comparative methods. We find significant relationships between viviparity and lower temperatures during the warmest (egg‐laying) season, strongly supporting the cold‐climate hypothesis. Remarkably, we also find that viviparity tends to evolve more frequently at tropical latitudes, despite its association with cooler climates. Our results help explain this and two related patterns that seemingly contradict the cold‐climate hypothesis: the presence of viviparous species restricted to low‐elevation tropical regions and the paucity of viviparous species at high latitudes. Finally, we examine whether viviparous taxa may be at higher risk of extinction from anthropogenic climate change.     

THE ALLIUM INFLORESCENCE: SOME SPECIES OF THE SECTION MOLIUM

Mann , Louis K. (U. California, Davis.) The Allium inflorescence: some species of the section Molium. Amer. Jour. Bot. 46(10): 730–739. Illus. 1959.—The inflorescence is described for Allium neapolitanum, A. roseum, A. hirsutum, A. subhirsutum, A. zebdanense and A. erdelii, species of Molium, a Mediterranean section of the genus. Inflorescence structure is similar among these 6 species. The single spathe appears to consist of 4 coalesced bracts, each of which bears in its axil a bostryx (helicoid cyme) of 3–7 flowers. Central to these 4 peripheral bostryxes are several smaller ones which differentiate later and decrease in flower number toward the inflorescence center. Bracts and bracteoles (prophylls) are generally absent within the spathe. The inflorescence is not radially symmetrical and the bostryx on the side opposite the uppermost foliage leaf differentiates and develops first. Among the peripheral bostryxes there is a definite sequence of development and certain regularities of coiling (clockwise vs. counterclockwise). Within each bostryx the flowers open in a strict sequence from oldest to youngest; among the bostryxes, the 4 peripheral ones flower first, usually starting with the first bostryx to be differentiated. The central bostryxes flower last.