Reproductive barriers between two sympatric beetle species specialized on different host plants

Abstract

Knowledge on interspecific pre‐ and post‐zygotic isolation mechanisms provides insights into speciation patterns. Using crosses (F1 and backcrosses) of two closely related flea beetles species, Altica fragariae and A. viridicyanea, specialized on different hosts in sympatry, we measured: (a) the type of reproductive isolation and (b) the inheritance mode of preference and host‐specific performance, using a joint‐scaling test. Each species preferred almost exclusively its host plant, creating strong prezygotic isolation between them, and suggesting that speciation may occur at least partly in sympatry. Reproductive isolation was intrinsic between females of A. fragariae and either A. viridicyanea or F1 males, whereas the other crosses showed ecologically dependent reproductive isolation, suggesting ecological speciation. The genetic basis of preference and performance was at least partially independent, and several loci coded for preference, which limits the possibility of sympatric speciation. Hence, both ecological and intrinsic factors may contribute to speciation between these species.

Assortative mating between two sympatric closely-related specialists: inferred from molecular phylogenetic analysis and behavioral data

“…Although the heterospecific mating was not detected for AV males, the males of AF occasionally mated with sibling species females under both no-choice and choice conditions in the laboratory (Table 3–5). Furthermore, previous cross-breeding experiments showed that hybrid F 1 with AV mother and three backcrosses (F 1 ♀ × AF♂, F 1 ♀ × AV♂, AV♀ × F 1 ♂) were viable, while F 1 with AF mother and one backcross (AF♀ × F 1 ♂) were inviable, suggesting asymmetric post-mating isolation between AF and AV due to cytoplasmic incompatibility1920. These results are consistent with asymmetric mitochondrial introgression estimated by coalescent analysis.…”

“…Considering the incomplete post-mating reproductive isolation181920, we suggest near perfect assortative mating has evolved. Differing from many studies in which allochronic factors play an important role in reproductive isolation and speciation41, allochronic isolation contribute little in the present system.…”

“…Pre-mating isolation due to host plant use may be occurring in the flea beetle genus Altica , a system that is becoming a model of host plant specialization and ecological speciation17181920. Many beetles in Altica are specialists, which feed and lay eggs on a limited range of plants.…”

“…Among them, Altica fragariae Nakane (hereafter ‘AF') is an oligophagous species with five recorded host plants belonging to the family Rosaceae, and Duchesnea indica (Andrews) Focke is the primary host plant in the field. Altica viridicyanea (Baly) (hereafter ‘AV') is a monophagous species that feeds on Geranium nepalens (Sweet) (Geraniaceae)181920. The beetles do not feed and oviposit on each other's host, even under no choice conditions, and the larvae do not develop on alternative host plants, indicating extreme host plant specificity1819.…”

“…The backcross between AF females and F 1 (AV♀ × AF♂) males is inviable, whereas all other three backcrosses (F 1 ♀ × AF♂, AV♀ × F 1 ♂, F 1 ♀ × AV♂) and F 2 generation are viable. Furthermore, these viable offspring can develop on one or both host plants with high survival rates181920. Interestingly, hybrid F 1 and backcrosses feed on both plants under choice conditions18.…”