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.     

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.     

THE FUNCTION OF CALL ALTERNATION IN ANURAN AMPHIBIANS: A TEST OF THREE HYPOTHESES

Males of many species of anurans alternate calls with those of their neighbors in a chorus. This pattern of calling reduces signal interference and may: 1) facilitate intermale spacing because males can better gauge the intensity of neighbors' calls if these calls do not overlap with their own; 2) help preserve species-specific temporal information in calls required to attract females; and/or 3) make it easier for females to localize males in the chorus. I tested these hypotheses with three species that exhibit call alternation, Hyla crucifer, H. versicolor, and H. microcephala. Males of all three species gave more aggressive calls to high-intensity synthetic stimuli that alternated with their calls than to those that overlapped their calls. These results support the first hypothesis. Results of four-speaker female choice experiments using alternating and overlapping calls indicate that preservation of signal integrity also is important in H. versicolor and H. microcephala, species that have fine-scale temporal information in their calls. However, the third hypothesis was not supported; females failed to discriminate among alternating and overlapping calls if the problem of signal disruption was eliminated or irrelevant.     

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.     

THE MORPHOLOGY OF THE INFLORESCENCE OF RAGWEEDS (AMBROSIA-FRANSERIA : COMPOSITAE)

Payne, Willard W. (U. Michigan, Ann Arbor.) The morphology of the inflorescence of ragweeds (Ambrosia-Franseria: Compositae). Amer. Jour. Bot. 50(9): 872–880. Illus. 1963.—The ragweeds possess an inflorescence which is highly modified in relation to anemophily. Several trends from primitive to specialized character expressions may be seen in the morphology of the unisexual capitula and florets. An evolutionary scheme is presented whereby the catkinlike, acropetally maturing, staminate spike and the sessile, centrifugally developing, fruiting involucre cluster are derived from basic inflorescence types within the Compositae. Morphological evidence suggests that: (1) the taxa Ambrosia and Franseria should be combined under the older generic name Ambrosia; (2) the ragweeds and their relatives as a group appear to occupy a position intermediate between the Heliantheae and Anthemideae, as currently delimited within the Compositae.     

ONTOGENY OF THE INFLORESCENCE IN CHENOPODIUM ALBUM

Gifford , Ernest M., Jr ., and Herbert B. Tepper . (U. California, Davis.) Ontogeny of the inflorescence in Chenopodium album. Amer. Jour. Bot. 48(8): 657–667. Illus. 1961.—Chenopodium album, a short-day plant, was induced to flower by subjecting it to successive cycles of 7 hr light and 17 hr darkness. After 4 inductive days, the first macroscopic change is evident in the appearance of precocious axillary bud primordia. After 5–6 days, a primordial inflorescence is visible, and after 7–8 days a terminal flower appears on the main inflorescence axis. The vegetative apex has a biseriate tunica, the cells of which are larger than those of the corpus. The cells of the tunica stain lighter, possess larger nucleoli, and are more vacuolate than cells of the subjacent corpus. After photoinduction, the tunica-corpus organization is maintained, and after 4 short-days, the shoot apex possesses a mantle of 3–4 layers of cells because there are few periclinal divisions in the cells of the outer corpus. The cells of the mantle stain uniformly and are more chromatic than those of the underlying tissue. Mitotic activity was recorded in the upper 40-μ segment of the apex. In the vegetative apex, mitotic activity is greater in the lower portion of the segment. Following photoinduction, mitoses increase throughout the apex until a maximum is reached on the 4th day. Also, the plastochronic interval decreases after photoinduction. Nucleoli of cells of the corpus enlarge following induction until all nucleoli of the apex are nearly equal. Included in the paper are discussions of the general morphological differences between vegetative and flowering shoots.     

EXPERIMENTAL STUDIES OF THE EVOLUTIONARY SIGNIFICANCE OF SEXUAL REPRODUCTION. V. A FIELD TEST OF THE SIB-COMPETITION LOTTERY HYPOTHESIS

Sexually and asexually derived tillers of Anthoxanthum odoratum were planted directly in the field to test the hypothesis that competition among groups of sexual and asexual siblings favors the maintenance of sexual reproduction in populations. The results showed a substantial fitness advantage for sexual tillers. However, in contrast with the models, the advantage of sex did not increase with increasing numbers of colonists in the patch, there were multiple survivors among colonists, and an advantage was observed even for singly planted tillers. When a truncation-selection scheme was imposed ex post facto on the data, the relative performance of sexual tillers was similar to that predicted by the Bulmer (1980) model, suggesting that sib-competition models fail due to the violation of the assumption of truncation selection. The advantage of sex was not correlated with the presence of other species, total percentage cover, or species diversity, although sites where sex was favored were physically clustered.