Sexual Isolation Between Two Sympatric Types of the Butterfly Eurema hecabe (L.)

Abstract

In Japan, the butterfly Eurema hecabe (L.) consists of two separate types. These types differ in wing fringe color (brown or yellow), which is linked with expression of seasonal wing morphs and with ability for host-plant use. It was recently found that these two types occur together on the subtropical Okinawa-jima Island (one of the Ryukyu Islands). In the present work we studied whether sexual isolation may exist between these sympatric types of E. hecabe on Okinawa-jima, using caged conditions. When living females (days 0, 1, and 7) were presented to males of the same or different type, females of the brown type received males of the same type and responded with a refusal posture to those of the yellow type. Females of the yellow type received males of the same type at all ages tested. However, when they were exposed to males of the brown type, the proportion of females showing a refusal posture changed with female age. The proportion was high at day 7 but relatively low at day 0 or day 1. When model females of the brown and yellow types were simultaneously presented to males of each type, males courted not only the model females of the same type, but also those of the different type. The results suggest that (1) sexual isolation exists between these types of E. hecabe on Okinawa-jima, although it is not perfect; (2) this isolation is mediated by female recognition but not male recognition; and (3) the sexual isolation system between two types is asymmetrical, depending on the female age after adult eclosion.