Greater Reproductive Capacity Research Articles

The Introduction and Diffusion of the African Honeybee in South America

In 1956 scientists introduced the African honeybee, Apis mellifera scutellata , into Brazil in an effort to improve honeybee genetic stock in that country. Almost immediately the bee escaped from captivity, hybridized with existing honeybee populations, and began diffusing over the South American continent. The bee diffused rapidly, eliminating existing honeybee populations through aggression, competitive advantage in resource exploitation, greater reproductive capacity, and gene flow. By the mid-1980s, the hybrid bee had spread over much of the continent, but some areas, notably south of 34 degrees south latitude, remained free of the hybrid because of its inability to adapt to environmental conditions, especially the cooler temperatures, found there. With the probable arrival of this hybrid honeybee in northern Mexico and the southern United States within the next five years, it behooves North Americans to study carefully the South American experience in order to better understand and prepare for the arrival of this bee in North America.

Three clonal types of keratinocyte with different capacities for multiplication.

Colony-forming human epidermal cells are heterogeneous in their capacity for sustained growth. Once a clone has been derived from a single cell, its growth potential can be estimated from the colony types resulting from a single plating, and the clone can be assigned to one of three classes. The holoclone has the greatest reproductive capacity: under standard conditions, fewer than 5% of the colonies formed by the cells of a holoclone abort and terminally differentiate. The paraclone contains exclusively cells with a short replicative lifespan (not more than 15 cell generations), after which they uniformly abort and terminally differentiate. The third type of clone, the meroclone, contains a mixture of cells of different growth potential and is a transitional stage between the holoclone and the paraclone. The incidence of the different clonal types is affected by aging, since cells originating from the epidermis of older donors give rise to a lower proportion of holoclones and a higher proportion of paraclones.

Consequences of limited seed dispersal within simulated annual populations.

Theoretical analyses of the consequences of dispersal have been almost entirely limited to studies of colonization. A simulation model is constructed and analyzed to study the effects of dispersal within local plant populations. Rules for the growth and reproduction of individuals are deterministic and related only to competition with other plants. Rules for the dispersal of individual seeds are stochastic; the generalized seed shadow is negatively exponential. The analysis has several interesting results. 1) In small, dense populations of annuals, mortality and growth behavior are significantly affected by small-scale changes in dispersal. 2) Stable population size, at any life stage, is not environmentally fixed (e.g. by the number of safe sites) but is also a function of reproductive capacity and dispersal. 3) There is a minimum productive capability which must be exceeded for a population to be able to respond to increased dispersal. The responsive range of the parameters may be related to the grain of safe site distribution. 4) When the total number of seeds per generation is large, high dispersal decreases the number of germlings that are able to establish. 5) For plants that reach the growing stage, however, expected reproduction can exceed that of genotypes with greater reproductive capacity but less dispersal. 6) An uneven distribution of seeds is essential to a spatial patterning of mortality which can deter competitive extinction.

The introduction of the alien amphipodGammarus tigrinus Sexton, 1939 (Crustacea, Amphipoda) in The Netherlands and its competition with indigenous species

In oligohaline watersGammarus tigrinus has a much greater reproductive capacity than any of the three indigenous (sub)speciesG. d. duebeni, G. zaddachi orG. p. pulex. The most important factors that enable this recently introduced species to compete the local species are: (a) its favourable reproductive period during the warm summer months, (b) its very short egg-incubation period, and (c) its very short time needed to reach sexual maturity.

Growth and Reproduction of Eragrostis Tenella in Pastures on the Upper Gangetic Plain

The reproductive capacity and growth forms of Eragrostis tenella Roem et Schult are directly influenced by the intensity and duration of light, the amount of exchangeable calcium in the soil, and particularly by grazing. Although the species grows under trees, it achieves its greatest height, spread, fresh weight, and seed output in full sunlight. Growth pattern is irregular in response to calcium, but maximum seed production is achieved in soil containing about 14.14 meq% exchangeable calcium. Grazing or clipping, far from diminishing the seed supply, seems actually to increase the total seed output, since frequently clipped plants and over—grazed fields show the greatest reproductive capacity, a fact which is significant for either the control or deliberate encouragement of E. tenella.

Balanced Polymorphism in Dactylis glomerata Sub-species woronowii

A CASE of balanced polymorphism, involving a recessive gene causing albinism, in Dactylis glomerata sub-species judaica has been reported by Apirion and Zohary1. What appears to be the same phenomenon is now reported for another diploid sub-species, woronowii, a seed-sample (No. 24171) of which was made available to me by the Division of Plant Industry, C.S.I.R.O., Canberra. Albino seedlings appear with a frequency of 13.5 per cent in this sample, and it may be taken that one is dealing with a recessive allele which, because of the lethality it confers in the homozygote, is present in unexpectedly high proportion. Evidently some superiority is attached to the heterozygous condition, which probably displays itself through a greater competitive ability in the seedling stage or a greater reproductive capacity when mature. However, because of the changed environmental circumstances in which the material is now being grown this selective advantage may be altered or lost. These possibilities are being investigated.

XII.—The Experimental Analysis of the Growth of an Insect Population

Human populations have afforded favourite material for analysis by statisticians, and others, interested in mathematical theories of population-growth. Experimentally, however, human beings are not ideal biological material, so that other animals, such as protozoa, insects, and mammals, have been used. On account of their great reproductive capacity, rapid response to changes in their environment, and ease of handling in the laboratory, certain insects are very suitable for this type of work. The enormous reproductive capacity of insects has been the subject of comment by many biologists, but we are still comparatively ignorant of the exact rôle of the various factors involved in the limitation of their numbers.