Specificity and daily cycle of release of sex pheromones in aphids: a case of reinforcement?

Abstract

Speciation by reinforcement, the process by which characters that increase assortative mating will he favoured because of selection against the production of less fit hybrids, is a much criticized evolutionary scenario, supported by little evidence. Previous studies have focused on hybrid zones, i.e. where different populations come into contact after diverging in allopatry, which might not be the optimal situation in which to look for reinforcement. We argue that reinforcement is more likely to occur in sympatrically diverging populations, like host races of phytophagous insects. In an attempt to demonstrate this we studied mate recognition of different life cycle forms, host races, sister species and more distantly related species of the aphid Cryptomyzus. All but one of these taxa mate on the same host plant, thus their mate recognition is subjected to similar ecological conditions, i.e. extrinsic selection pressures. The males of the life cycle forms and host races were not differentially attracted in an olfactometer by their respective females, which all released sex pheromones during the same period of the day. The females of the sister species, however, released sex pheromones at different times of the day, one 'calling' in the morning and the other in the afternoon. This difference is corroborated by a corresponding pattern of activity in the males, and a marked preference of the males for conspecific pheromone. It is argued that these sister species originated sympatrically following a shift in summer host plant utilization. As their hybrids are less viable than the F1 of intraspecific crosses reinforcement might have been a major force in the development of specific mate recognition, i.e. the specificity and difference in the daily cycle of release of the sex pheromones.

Natural hybridisation between Aphis grossulariae and Aphis schneideri: morphological evidence (Sternorrhyncha: Aphididae)

“…microsatellites and mitochondrial DNA techniques, see Hales et al, 1997;Sunnucks et al, 1997) would help to evaluate the degree of introgression between natural populations of these species. Natural isolating mecha nisms such as sex pheromone specificity, circadian rhythm of sex pheromone release, and other aspects of possible natural specific mate recognition system should be also studied Guldemond & Dixon, 1994;Thieme & Dixon, 1996). Morphological features of different morphs did not coincide within the same sample.…”

Experimental hybridisation between Aphis grossulariae and Aphis triglochinis (Sternorrhyncha: Aphididae)

“…Natural prezygotic isolating mecha nisms might be rather sophisticated and fragile Guldemond & Dixon, 1994;Thieme & Dixon, 1996), and be easily circumvented by the experi mental procedure. So, information on successful experi mental hybridisation reinforces the need for the study of natural isolating mechanisms between the species involved, such as sex pheromone specificity, the circadian rhythms of sex pheromone release and male activity, and other aspects of possible species-specific mate recogni tion systems.…”

Physiological determinants of male mating performance in aphids (Hemiptera: Aphididae)

“…Females have a known circadian cycle of pheromone release from glands on the hind tibiae (see for example Guldemond & Dixon, 1994;Thieme & Dixon, 1996, Goldensaz & McNeil, 2003. The experiments described in the present paper were probably unaffected by pheromone attraction, since the aphids were confined in small containers which would become saturated with pheromone if it was released.…”