SPECIES OF EPILACHNA LADYBIRD BEETLES

The present paper reviews current studies on various aspects of reprod uctive isolation in a series of closely related phytophagous ladybird beetles called the Epilachna vigintioctomaculata complex, which is com posed of two groups and four species: group A comprising E. vigintioct omaculata, and group B comprising one northerly distributed species, E . pustulosa, and two southerly distributed species, E. niponica and E. yasutomii. Being broadly sympatric from middle through northern Japan , the two groups are reproductively isolated by a combination of sever al factors, each of which functions as an incomplete barrier to gene f low: difference of host plants, weak sexual isolation, and low hatchin g rates of eggs produced by interspecific matings. Conspecific sperm p recedence further lessens the probability of producing hybrids in mixe d populations. By contrast, the two sympatric species of group B, E. n iponica and E. yasutomii, are reproductively isolated from each other solely by fidelity to different host plants. However, there are no eff ective barriers to gene exchange between either of these two species a nd the northerly distributed E. pustulosa. These situations have relev ance to various controversial issues in evolutionary biology, covering the mode of speciation in host specific insects, role of postinsemina tion barriers to fertilization, reinforcement of reproductive isolatio n, and treatment of allopatric populations in speciation studies. The E. vigintioctomaculata complex thus offers a rare opportunity to exten d our understanding of the nature of animal species and their origin.