Timothy Prout

The joint effects of the release of sterile males and immigration of fertilized females on a density regulated population.

Abstract The joint effects are studied of the release of sterilized males and immigration of mated females on a population whose discrete generation recursion is, N′ = N( RK K + (R − 1)N ) where, R ≡ reproductive potential, K ≡ equilibrium. This form of growth is derived from life history considerations so that the impact of the release of sterile males on immatures and on adults can be compared. When the migration parameter and the sterile male release parameter are small, the system has three internal equilibriums (the middle one being unstable). Increase in immigration or in release results in one stable equilibrium. The practical conclusion is that migration must be very small in order for the release of sterile males to be effective on suppressing numbers of adults, while more migration can be tolerated if, as in many agricultural pests, immature stages are the object of concern.

Environmental Effects on Remating in Drosophila melanogaster

The effects of density and food on remating were investigated using Drosophila melanogaster. The frequency of remating was unaffected by density for some combinations of fly strains but was reduced at low relative densities for other combinations. Until females had used most of their stored sperm, remating was less likely when food was absent or contact with food was prevented. Food availability had little effect on the incidence of remating once stored sperm were depleted and had no effect on initial virgin mating frequency. This study indicates that environmental factors can have a substantial direct influence on the frequency of remating in populations of Drosophila melanogaster.

Seminal fluid causes temporarily reduced egg hatch in previously mated females

In Drosophila, male accessory gland fluid (seminal fluid) has multiple effects on the females reproductive efficiency. Here, we show the effect of seminal fluid on rate of egg hatch immediately following mating. Singly mated females were remated to two classes of sterile males, one with seminal fluid and one without seminal fluid. Transfer of seminal fluid results in a strong reduction in egg hatch shortly after the mating. Also, it is shown that remating with normal males causes an immediate reduction of egg hatch followed by recovery to normal egg hatch. In all cases, unhatched eggs contained no sperm. These results are consistent with a role for seminal fluid in sperm competition, mediated by incapacitation or inefficient use of resident sperm.

OBSERVATIONS ON STRUCTURAL REDUCTION IN EVOLUTION

The problem of structural reduction in evolution constitutes a special case of the more general problem of seemingly non-adaptive evolutionary changes. As Brace points out, many of the characters which differentiate species or higher categories appear trivial in that it is difficult to imagine how such characters in themselves directly contribute to the differences in adaptation. Numerous examples of such characters may easily be found by thumbing through analytic keys found in field guides to identification. Dobzhansky (1956) in discussing this same problem presents an impressive list of apparently trivial characters differentiating Drosophila species. The inability to understand or even imagine the direct adaptive significance of such structures immediately causes one to ask whether some evolutionary changes occur by means other than direct selection. The cases of structural reduction raise this question for the same reason. It is difficult to imagine the adaptive significance of the loss of the anthropoid tail or loss of the lateral digits in the horse lineage. If the adaptive significance of such changes were evident, then non-selective mechanisms for these changes

The Delayed Effect on Fertility of Preadult Competition: Two-Species Population Dynamics

The delayed effect on fertility of preadult crowding is incorporated into a model of competition between two species. Using the symmetric case of the exponential version of the Lotka-Volterra function, equilibrium solutions are examined for two life stages: adults, N̂, and eggs, n̂. It is found that, when looking at adult numbers, this delayed effect can give the appearance of competitive interactions quite different from the true relationships and, in the extreme, can result in apparent mutualism between strong competitors.