Genetic Modification of Host Acceptance by a Seed Beetle, Callosobruchus Maculatus (Coleoptera: Bruchidae)

Abstract

Successful host shifts by herbivorous insects may require the modification of multiple larval and adult traits. The seed beetle Callosobruchus maculatus (F.) (Coleoptera: Bruchidae) rarely attacks lentil (Lens culinaris Medikus), which is distantly related to its typical hosts. In a previous study, larval survival in lentil seeds increased from <2 to >85% in fewer than 20 generations of laboratory selection. However, lentil is also a poor oviposition host; lifetime fecundity on lentil was initially less than a third of that on the ancestral host [ mung bean, Vigna radiata (L.) R. Wilczek ] and one fourth of females did not even recognize lentil as a potential host. This study examined the genetic lability of host acceptance. We performed both quasi-natural selection, in which replicate lines switched to lentil were compared with those remaining on mung bean, and artificial selection, in which lines were established using females from the base population that exhibited especially low or high acceptance of lentil during a short-term assay. After only five to eight generations of quasi-natural selection, lifetime fecundity on lentil was 2 to 3 times higher in the lentil lines than in the mung bean lines. Lentil-line females also accepted the novel host sooner after adult emergence. Similarly, a single generation of artificial selection was sufficient to increase oviposition on lentil in the acceptance line. Host acceptance was not genetically correlated with larval survival, which remained <2% in lines artificially selected for either high or low acceptance of lentil seeds. Although modification of oviposition behavior in this study was not nearly as striking as the increase in larval survival reported previously, the C. maculatus population possessed enough standing genetic variation in both larval physiology and adult behavior to permit rapid adaptation to a very poor host.

Specificity of adaptation to a novel host plant by a seed beetle

“…Instead, lentil-adapted females appear to have evolved a lower level of specificity in their oviposition decisions (Mercader & Scriber, 2007;. Lower host specificity may be a predictable consequence of spending 60 or more generations on a host that initially elicits little or no egg-laying (Messina et al, 2009b). Variation in host specificity produced by Table 2).…”

“…To reduce any non-genetic, rearing-host effects, subcultures of the three lentil lines were reverted to mung bean for a generation before the assays (Magalhães et al, 2011). Thus, all test females encountered only mung bean both as larvae and as newly emerged adults (Messina et al, 2009b;Messina & Peña, 2012). Cultures of the four lines (M and L1-3) were first sieved to remove all adults that had already emerged.…”

“…Lentil seeds also elicit little oviposition. Initially, more than 25% of females laid zero eggs over their lifetimes when presented only lentil seeds, but acceptance of lentil increased two-to three-fold in the lentil lines (Messina et al, 2009b). Crosses between the lentil lines and the ancestral line maintained on mung bean [Vigna radiata (L.) Wilczek (Fabaceae)] suggested largely additive inheritance of increased oviposition on lentil, whereas the more remarkable increase in larval performance was due to both additive and non-additive genetic components (Messina & Jones, 2011;Messina & Peña, 2012).…”

Egg-Dumping Behavior is Not Correlated with Wider Host Acceptance in the Seed Beetle Callosobruchus maculatus (Coleoptera: Chrysomelidae: Bruchinae)

“…Contrary to the hypothesis, however, African females did not show greater acceptance of lentil (Table 1 ), which represented both a novel and less preferred host. Before we conducted the selection experiments that eventually yielded the L lines in this study, a similar comparison was performed using the Utah strains of the African and Asian populations (Messina et al 2009b;unpublished data). Although these strains were even more divergent in the tendency to dump eggs (Messina et al 2007), they too did not differ in the number of eggs laid on lentil (respective means [ ± SE] were 10.8 ± 1.4 and 10.2 ± 1.1; Í = 0.32, df = 175, P = 0.75).…”

“…Gultures were first sieved to remove already emerged adults, and newly emerged pairs were collected within 1 h. On the first day, pairs were placed in empty 60-mm dishes to estimate egg dumping. We recorded the number of eggs laid per female after 6 d. Because Einy differences in egg dumping between lines could simply reflect differences in fecundity, pairs collected on the second day were placed in dishes containing »«100 seeds of mung bean, a highly preferred host, for 6 d. Pairs collected on the third day were placed in dishes containing =»100 lentil seeds, again for 6 d. This treatment was meant to confirm that the L and M lines differed in their acceptance of the novel host (Messina et al 2009b). None of the 720 test females (four lines X 3 host treatments X 60 replicate females per treatment) had prior egg-laying experience, because they were collected within an hour of adult emergence.…”