Asymmetry in the reproductive interference between two closely related species of spider mites, Panonychus citri and Panonychus osmanthi (Prostigmata: Tetranychidae)

An incomplete species recognition system may cause reproductive interference. Interspecific mating between Panonychus citri and Panonychus osmanthi produced apparently normal F₁ hybrid females. However, the F₁ females laid few eggs, and the produced eggs did not hatch (suggesting hybrid breakdown). Several common host plants of P. citri and P. osmanthi have been determined, including Japanese pear, peach, and citrus. However, P. osmanthi has been found only on its particular host plants in the genus Osmanthus. To assess the potential for reproductive interference between P. citri and P. osmanthi, we evaluated the competitiveness (1 ♀ × 2 ♂♂) and preference (2 ♀♀ × 1 ♂) of males in mating and guarding behavior and assessed the effect of second mating on offspring paternity using Japanese pear leaf disks. We found that P. citri males were superior competitors. Moreover, the species recognition ability of P. citri males was lower than that of P. osmanthi males, and frequent occurrence of reproductive interference should therefore be expected. In a mixed population (2 ♀♀ × 2 ♂♂), 41% of females copulated with heterospecific males, although the mating superiority of P. citri was not proven. The ratio of effective second mating with conspecific males after heterospecific mating was higher in the crosses to P. citri females (77.8%) than P. osmanthi females (14.3%). Therefore, P. citri females had more chance than P. osmanthi females to recover from the fitness cost imposed by heterospecific mating through subsequent conspecific mating. To date, P. osmanthi has not been detected on host plants other than Osmanthus species. Reproductive interference by P. citri males may therefore narrow the range of host plants available for P. osmanthi.

Does heterospecific seminal fluid reduce fecundity in interspecific copulation between seed beetles?

Reproductive interference through mating between related species can cause fitness reduction and affect population dynamics of the interacting species. In experimental matings between two seed beetles, Callosobruchus chinensis and Callosobruchus maculatus, C. maculatus females, but not C. chinensis females, suffer from significant loss of fecundity when conspecific mating is followed by heterospecific mating. We hypothesized that male traits associated with sexual conflict, which are often harmful to females, pleiotropically affect fitness of heterospecific females through interspecific mating. We examined the effect of ejaculate of C. chinensis males on C. maculatus females as the cause of the fecundity loss in C. maculatus females due to interspecific copulation. We found that frequent interspecific copulation occurred between C. maculatus females and C. chinensis males, but not between C. chinensis females and C. maculatus males, resulting in frequent interspecific ejaculate transfer from C. chinensis males to C. maculatus females. However, injection of the extract from C. chinensis male reproductive organs into C. maculatus females did not significantly affect C. maculatus fecundity compared with saline injection, indicating that the effect of the heterospecific ejaculate transfer on fecundity is negligible. We suggest that other harmful male traits such as genital spines of C. chinensis males are mainly responsible for the fecundity reduction in C. maculatus females that have experienced interspecific mating.

RFLP analysis of ribosomal DNA in sibling species of spider mite, genus Panonychus (Acari: Tetranychidae)

Ribosomal DNA (rDNA) restriction fragment length of spider mites was analysed in three sibling species of genus Panonychus (P. mori, P. citri and P. ulmi) and Tetranychus urticae. Seventy-one fragments were detected by enhanced chemiluminescence (ECL) after Southern hybridization with two genomic rDNA probes of mouse. Percentages of shared fragments within species were larger than those among species, indicating that the variation of rDNAs was useful in phylogenetic studies of spider mite species. In a phenogram based on differences of the rDNA fragment length, the relationship among Panonychus species was compatible with the results of analysis based on the morphological characters.