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between-population genetic variation for each trait (Messina, 1989), and

host-discrimination behaviour was also found to be heritable within pop-

ulations (Messina, 1993; Tanaka, 2000). The selection experiment used an

Asian strain with highly uniform egg laying and contest competition. This

base population was collected from a small host (mung bean) and was

consistently maintained at large population sizes on this host (Messina,

1991a).

The original population was divided into six lines. Three replicate

lines were maintained on the ancestral host, and three were transferred to

a larger, novel host, cowpea (Fig. 4.3; the cowpeas had approximately

three times more mass than mung beans). After more than 40 generations

on each host, we examined larval competitiveness in each line by estab-

lishing either one or two larvae per seed in both mung bean and cowpea.

We quantified host discrimination as the uniformity of egg distributions

produced by females that were each given 40 seeds. We used theUscore

of Messina and Mitchell (1989; see also Horng, 1997), which is not as

influenced as other dispersion indices by the number of eggs laid. This

index typically varies between zero, which signifies random egg laying

(no host discrimination), and one, which represents the most uniform

distribution possible. To reduce possible host-related, non-genetic,

maternal effects (Fox and Savalli, 1998, and references therein), we reared

all lines on mung beans for two generations before the test generation.

This step introduced a conservative bias because traits in cowpea-selected

Host Discrimination by Seed Parasites 77

Fig. 4.3. Schematic diagram of a selection experiment in which an ancestral

population ofCallosobruchus maculatuson mung bean (M) was divided into

replicate lines and maintained on the same host (M1–3) or transferred to cowpea

(C1–3). All lines were returned to mung beans for two generations to avoid

host-related maternal effects on the test generation.