The genetic basis as well as the mode of inheritance of polyandry in Spodoptera exigua HuÈ bner was studied in the laboratory by using a simple divergent selection experiment followed by F 1 reciprocal crosses, F 2 and backcrosses. There was an eective response to arti®cial selection for high (H line) and low (L line) female mating frequency with signi®cant separation of the lines by the second generation of selection. The mean female mating frequency in the parental generation (1.57 matings per female) reached plateaus of 2.50 and 1.25 matings per female in the H and L lines, respectively, after six generations of selection. Selection response becomes saturated at about 90% and 25% levels of polyandry (percentage females re-mating) in the H and L lines, respectively, and consequently monoand polyandric pure strains were not obtained. Polyandry levels in ospring from the H and L lines and their hybrids in F 1 , F 2 and backcrosses consistently indicate that female mating frequency was more or less proportional to the relative amounts of genes derived from the H and L lines. Such a clear pattern of hybrid responses, together with the gradual selective changes under arti®cial selection, suggests the involvement of a polygenic system. Female mating frequencies from progeny of the two reciprocal F 1 crosses were not signi®cantly dierent, which suggest that the trait was autosomally inherited. Moreover, female mating frequency of F 1 (pooled) progeny was not signi®cantly dierent from the mid-parental value, which suggest no dominance. The computation of the Cavalli's joint scaling test consistently con®rmed these results yielding values of d 0.51 0.10 and h 0.12 0.21. The broad sense heritability estimate was H 2 0.73. It is concluded that polyandry in S. exigua is a polygenic, autosomal heritable trait and that additive genetic variance is available for selection for female mating frequency. The implications of the genetic basis of polyandry are brie¯y discussed in the context of current theories about this crucial insect mating system. Keywords: heritability, inheritance, mating frequency, multiple mating, polyandry, polygenic control, Spodoptera exigua. IntroductionInsect species are usually classi®ed as monandrous or polyandrous if most females exhibit single or multiple mating, respectively. Polyandry is a crucial female behaviour because this inevitably results in sperm competition and may be accompanied by cryptic female choice. Knowledge of factors regulating polyandry is important in the study of the evolution and adaptive signi®cance of insect mating systems. Issues include evolutionary ecology, sexual behaviour, con¯ict of interests between sexes, mating bene®ts and costs, parental investment and female (ospring number and ®tness) or male (paternity) reproductive output and success (see Choe & Crespi, 1997; Birkhead & Moller, 1998; for recent extensive reviews).The occurrence of polyandry in insects has been shown to be determined by an array of environmental and morpho-physiological factors. From these, ...
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