Reproductive isolation and genetic divergence among populations of green lacewings of the genus {\it Chrysoperla} (Neuroptera:Chrysopidae)

Date of Completion

January 1991

Keywords

Biology, Entomology|Biology, Genetics|Biology, Zoology

Degree

Ph.D.

Abstract

This study investigated the importance of courtship songs in maintaining reproductive isolation between three distinct song morphs of green lacewings in the Chrysoperla plorabunda complex, as well as the degree of genetic divergence among populations. Males and females establish duets during courtship that culminate in copulation. The songs differ in temporal features and in the mode of presentation during duetting (one vs. several volley units). I asked the following questions: (1) Are the song morphs distinct biological species? (2) To what features of the songs do insects respond? (3) Do the song morphs hybridize in the laboratory and produce viable offspring? (4) If the three song morphs are different species, how much genetic divergence, if any, has occurred among them?^ Playback experiments showed that females of each morph responded significantly more to songs of their own morph than to songs of other sympatric morphs. These results demonstrated that courtship songs are strong premating barriers to reproduction that keep sympatric populations of green lacewings separate as true biological species. Experiments with synthetic courtship songs suggested that a frequency modulated signal is required to elicit a response from females of the P1 morph. These insects will answer songs that are slower than their own, but cannot duet with faster songs.^ There was a 30% reduction in larval viability from the homotypic crosses to the second generation of hybrids, when P1 morphs were crossed with P3 morphs. This revealed a postmating barrier to reproduction and the possibility of selection on song morphs to mate assortatively. Playback experiments with hybrid songs showed that hybrid individuals preferred hybrid songs over at least one of the parental songs, and the parental species were unresponsive to hybrid songs.^ Electrophoretic analysis revealed small Nei's genetic distances ($<$0.005) between P1 and P3 song morphs, and ($<$0.01) between the morphs and another species, C. downesi. These data suggest that all of these species have diverged very recently. In conclusion, the combined data from analysis of songs, behavioral experiments, laboratory hybridization, and genetic analysis support the hypothesis that the song morphs of the Chrysoperla plorabunda complex are distinct biological species. ^

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