ADAPTATION IN PANTOTHENATE-REQUIRING NEUROSPORA II. NUCLEAR COMPETITION DURING ADAPTATION

Davis , R. H. (U. Michigan, Ann Arbor.) Adaptation in pantothenate-requiring Neurospora. II. Nuclear competition during adaptation. Amer. Jour. Bot. 47(8) : 648–654. Illus. 1960.—The process of adaptation in pantothenate-requiring Neurospora was studied by the use of heterocaryons constituted of the pan-1 strain and a modified strain, pan-1 m, derived from it. Although pan-1-m homocaryons grow well on a pantothenate concentration on which pan-1 grows little or not at all, pan-1 nuclei often have a selective advantage in pan-1 + pan-1-m heterocaryons grown on the same medium. This results in non-adaptive changes in nuclear ratios and labile growth rates of the heterocaryons. Nuclear competition does not occur when pantothenate is not limiting to the growth of either homocaryon. The results are discussed, and related to past work on adaptation and nuclear ratio changes in Neurospora hetercaryons.     

THE COMPONENTS OF KIN COMPETITION

It is well known that competition among kin alters the rate and often the direction of evolution in subdivided populations. Yet much remains unclear about the ecological and demographic causes of kin competition, or what role life cycle plays in promoting or ameliorating its effects. Using the multilevel Price equation, I derive a general equation for evolution in structured populations under an arbitrary intensity of kin competition. This equation partitions the effects of selection and demography, and recovers numerous previous models as special cases. I quantify the degree of kin competition, α, which explicitly depends on life cycle. I show how life cycle and demographic assumptions can be incorporated into kin selection models via α, revealing life cycles that are more or less permissive of altruism. As an example, I give closed‐form results for Hamilton's rule in a three‐stage life cycle. Although results are sensitive to life cycle in general, I identify three demographic conditions that give life cycle invariant results. Under the infinite island model, α is a function of the scale of density regulation and dispersal rate, effectively disentangling these two phenomena. Population viscosity per se does not impede kin selection.     

一个竞争模型的一致持续生存

本文研究一个带有时滞的竞争模型的一致持续生存。首先证明了离散时滞不影响种群的一致持续生存,接着在种群增长率是周期的假设下讨论了正周期解的存在性,最后给出了连续时滞模型一致持续生存的充分条件。     

SPERM COMPETITION GAMES: A GENERAL MODEL FOR PRECOPULATORY MALE–MALE COMPETITION

Reproductive males face a trade‐off between expenditure on precopulatory male–male competition—increasing the number of females that they secure as mates—and sperm competition—increasing their fertilization success with those females. Previous sperm allocation models have focused on scramble competition in which males compete by searching for mates and the number of matings rises linearly with precopulatory expenditure. However, recent studies have emphasized contest competition involving precopulatory expenditure on armaments, where winning contests may be highly dependent on marginal increases in relative armament level. Here, we develop a general model of sperm allocation that allows us to examine the effect of all forms of precopulatory competition on sperm allocation patterns. The model predicts that sperm allocation decreases if either the “mate‐competition loading,”a, or the number of males competing for each mating, M, increases. Other predictions remain unchanged from previous models: (i) expenditure per ejaculate should increase and then decrease, and (ii) total postcopulatory expenditure should increase, as the level of sperm competition increases. A negative correlation between a and M is biologically plausible, and may buffer deviations from the previous models. There is some support for our predictions from comparative analyses across dung beetle species and frog populations.     

COMPETITION FOR PHOSPHORUS BETWEEN DESMIDS1

During competition for phosphate in continuous cultures, Cosmarium subcostatum Nord. routinely displaced Staurastrum paradoxum Meyer. The rate of displacement was independent of cell density between 100 and 6000 cells mL^?1. This suggests that competition for nutrients is important over a wide range of naturally occurring cell densities. C. subcostatum had higher saturated rates of phosphate uptake but also higher half saturation values for uptake. As a result, the two desmids were similarly able to take up phosphate at low concentrations. The competitive advantage of C. subcostatum lay in its greater yield per unit of phosphorus. Growth of the two algae in shared medium in a dual-chamber chemostat had no effect on uptake or yield characteristics.     

植物竞争研究进展

竞争系指两个以上有机体或物种间阻碍或制约的相互关系。它是塑造植物形态、生活史的主要动力之一;并对植物群落的结构和动态具有深刻的影响。因其在生态学的中心地位,生态学家已从不同的侧面研究了这一复杂的生态学现象;生态学也因此而得到了发展。然而,人们对竞争的理解不尽一致,因而导致了概念上的混乱,平行研究相对缺乏、不同研究间的比较困难,从而阻碍了学科的发展。本文试就植物竞争的概念、竞争理论、竞争研究的实验方法、影响竞争能力的主要因素、种内和种间竞争对种群和群落的影响,如竞争与物种共存等进行综述;我们在总结研究成就的同时,亦指出了现有研究的局限性。     

POLLEN COMPETITION AND RELATEDNESS IN HAPLOPAPPUS SECTION ISOPAPPUS (COMPOSITAE). II.

Pollen competition in mixed pollinations involving a time handicap was studied among the five aneuploid members of section Isopappus of Haplopappus. Foreign pollen was applied to the stigmas of a plant followed after a delay of 0–180 min by domestic pollen. Domestic pollen was prepotent over foreign pollen in all cases, but with increasing delay an increasing percentage of hybrids appeared in the progeny. The time interval required to produce 50% of hybrids in the progeny (at which time the two pollen types were competitively matched in potency) was determined for each cross. The interval is correlated with relatedness, with chromosome number, and with length of the style of the pollen parent. It can be used as an estimate of the potential frequency of hybridization in nature, with all other factors being equal.     

INTRAPOPULATION DIFFERENTIATION IN ANNUAL PLANTS. III. THE CONTRASTING EFFECTS OF INTRA- AND INTERSPECIFIC COMPETITION

The roles of intraspecific and interspecific competition in producing differentiation within populations of Veronica peregrina were studied in two populations under controlled, greenhouse conditions. In nature, each population spans an environmental gradient across the center and sides of a temporary, vernal pool in California. Individuals at the center are subjected to intense intraspecific competition produced by high densities (to 30 seedlings/cm²) generated by quasi-simultaneous germination (90% of seeds germinate in one week). Individuals at the periphery are subjected to interspecific competition with grasses, which shade out the Veronica 4–6 weeks after the onset of winter growth. I predicted that 1) when grown under immediate intraspecific competition in the greenhouse, offspring of plants from the central subpopulation (C) would perform better (i.e., grow larger and produce more seeds) than those from the periphery (P) and that 2) when grown under delayed interspecific competition provided by Agrostis tenuis and Lollium multiflorum, offspring of plants from the periphery would perform better than those from the center. Both predictions were confirmed. The center-periphery differences were pronounced and statistically significant in an undisturbed population (V-2), while in a population disturbed by yearly plowing (V-3), the differences tended to be consistent with those in V-2 but seldom significant. Distribution of variability tended to be positively skewed and/or leptokurtic in subpopulations grown under “foreign” competition (i.e., intraspecific for P plants and interspecific for C plants) but was normally distributed following exposure to “familiar” competition. Timing of competition affected many results. There were four additional significant differences between the central and peripheral subpopulations. 1) Germination rate: the faster rate in central plants can be advantageous under immediate intraspecific competition. The slower rate in peripheral plants can be advantageous under conditions of erratic and unpredictable soil moisture. 2) Response to nutrient competition: central plants were more sensitive to N-deficiency and peripheral plants were more sensitive to P-deficiency. 3) Allocation of biomass: central plants allocated a greater proportion of biomass to seeds, while peripheral plants allocated a greater proportion of biomass to leaves under all growing conditions. 4) Root elongation: at the seedling stage, central plants have longer roots, while at the adult stage, peripheral plants have longer roots (but not more root mass). Most components of this complex pattern of differentiation are interpretable in an adaptive context. Other results defy simple explanations and underline the importance of phenotypic plasticity, which was pronounced in the competition experiments.