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Study of the variation due to maternal age in Hylemya Antiqua Goth, Georgia J.

Abstract

In 1928, Jennings and Lynch found that the individuals of a clone of rotifers were not intrinsically alike, but varied in fertility and life span depending on the nature of the eggs from which they came. Lansing (1953) suggested that this diversity was due to an aging factor transmitted to the offspring through the eggs of middle-aged and old mothers. He thought that this aging factor would accelerate the rate of aging in the offspring. This led to a number of similar studies in which attempts were made to demonstrate a "Lansing-effect" in other species. However, there was still a need for a better understanding of the extent to which maternal aging might be a source of variability among offspring, and the effect of this variability on population dynamics. This was the goal of the present research project. Maternal-age effects on a variety of life history traits were studied in populations of Hylemya antiqua, the onion root maggot, raised in the laboratory under controlled conditions. A number of differences were elucidated. When the mother is young, she produces her most reproductively successful offspring. These offspring have a high survival probability until they reach mid-life, at which point their mortality rate begins to increase more rapidly. Nevertheless, they have a long mean expectation of life. They have the highest net fecundity (i.e., average number of eggs produced per female per 48 hours) and at certain ages (11 to 30 days) show the highest rate of egg production. A higher percentage of the total offspring they produce throughout their reproductive spans are female. These qualities all contribute to give them the highest innate capacity-of natural increase (1.5009) in comparison with later born offspring. As the mother passes into middle-age, her offspring display reduced reproductive capabilities, but their overall survival capabilities are maximal. These young show the lowest sustained mortality rate throughout life and have as long a mean expectation of life as their early-born sisters. They are also the hardiest in terms of their ability to survive food stress-a greater proportion are able to survive starvation longer. Their net fecundity is intermediate between that for early-born and that for late-born offspring, as indicated by their rate of increase (1.3712). A mother in old age produces her least viable offspring (i.e., those with the highest mortality rate and the shortest mean expectation of life), and her least fecund offspring (both in terms of net fecundity and rate of egg production). These offspring however have the fastest turn-over rate (mean generation time). Nevertheless, they make the least contribution to succeeding generations, with a rate of increase of 1.2874. These maternally influenced differences in rate of increase have a remarkable effect on population growth; e.g., after ten generations in a constant environment a population of early-born offspring could potentially be 10 times as dense as a late-born population. A young mother produces either her largest or her smallest offspring (depending on provenance) in terms of pupal size. Age-specific fecundity and mortality differences, however, are not related to these size differences. Finally, maternal age also affects the dispersal ability of larvae and the activity of adult females. A certain percentage (7%) of larvae from middle-aged and older mothers show an innate tendency to disperse, whereas those from young mothers tend to remain on the original food source. Trends in adult activity do not coincide with these larval activity differences. As adults, mid-born females are slightly less active than either early- or late-born females. Maternal age, therefore, is a source of variability among offspring in this species. Populations of Hylemya antiqua exhibit overlapping generations, and at any one time will contain ovipositing females of different ages. When a female alters the characteristics of her offspring as a function of her age, it not only represents a strategy of spreading the risk of extinction of her own genetic complement, but it introduces phenotypic variation, thereby representing a strategy for survival of the population as a whole.

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