Project: Drosophila behavioral genetics

People: J. Gleason, B. Sanderson, F.  Ighoyivwi
Description:
Speciation is responsible for the diversity of life on earth.  The process of speciation requires the evolution of reproductive isolation between populations.  Differentiation can result from postmating isolation (hybrids are sterile or inviable) or premating isolation (species do not mate).  Although the genetics of postmating isolation have received much attention, the genetics of premating isolation have not.

We are studying courtship traits that contribute to premating isolation.  Drosophila simulans and D. sechellia differ in two courtship signals and preferences.  The first signal, courtship song, is produced by the male to stimulate the female.  Females greatly prefer the song of their own species and only reluctantly mate in the presence of a different species’ song.  The second signal is a contact pheromone in the form of a cuticular hydrocarbon.  D. sechellia females have a different major cuticular hydrocarbon than D. simulans females.  This affects sexual isolation asymmetrically because D. simulans males discriminate against D. sechellia females whereas D. sechellia males do not discriminate.
Two major approaches are being taken towards genetics analyses of these traits.  The first approach uses quantitative trait loci mapping and recombinant inbred lines to estimate the number of loci, their genomic locations and the direction of effect for both signals and preferences. The results will indicate the relative effects of drift and sexual selection on each trait and thereby provide information about processes of speciation.  The second approach uses introgression and recombinant inbred lines to isolate qualitative effects of genetic factors and to identify changes in gene expression using microarray analysis. 
Funding:  NSF

Project: Courtship song evolution in the D. saltans group

People: J. Gleason, E. Schmidt, J. Cooper
Description:
Courtship in many animal species consists of specific signals sent between the male and female prior to mating.  These signals provide species recognition and may be subject to sexual selection.  Courtship in Drosophila melanogaster starts with a male orienting towards the female, tapping her abdomen with his legs (probably to detect chemical cues through receptors in his feet), vibrating his wings, usually to produce courtship song, and licking of her genitalia.  Many other Drosophila species do the same behaviors as well as perform a variety of wing movements (Spieth 1952).  The behaviors can progress in any order, and behaviors can be repeated.  The male will often attempt copulation and, when the female acquiesces, copulation proceeds.

Courtship song has a large effect on female choice in D. melanogaster.  Male wing vibration produces two types of courtship song, sine song and pulse song.  Sine song is a low frequency hum, which has a stimulatory effect on the female.  Pulse song consists of a series of low frequency pulses .  The time between each pulse, the interpulse interval (IPI) is important for species mate recognition. IPI is highly species-specific in the D. melanogaster group.  Females mate much more quickly in the presence of conspecific song than in the presence of heterospecific song.  Although song is not absolutely necessary for mating, playing song partially restores the mating success of mute (wingless) males.

Courtship song has been studied in a limited number of Drosophila species groups.  Although the pattern of evolutionary change for the D. melanogaster species group is modification of IPI, other species groups have variation in patterns .  In particular, song patterns in the D. willistoni group can be used to identify species, however, they cannot be used to reconstruct the phylogeny of the group (Ritchie and Gleason 1995; Gleason and Ritchie 1998).  The lack of phylogenetic signal implies that there has been sexual selection for song, indicating that song may be important for mate choice (Gleason and Ritchie 1998).  Furthermore, from an analysis of the songs with premating and postmating isolation, it is clear that song evolves very quickly (Gleason and Ritchie 1998).  Such differentiation is not seen in all species groups.  In some species groups, species are not well differentiated , thus song divergence may result from genetic drift, indicating that song may not be as important for species recognition.

Project:  Signaling in D. nebulosa

People: J. Gleason, A. Vezeau
Description:

Project: Waxmoth behavioral genetics

People: Y. Zhou, M. Greenfield, J. Gleason (and others in the Greenfield lab)
Description:
The presence of genetic variation for male sexual traits such as mating signals remains a major problem in evolutionary biology.  Female choice often favors exaggerated mating signals and is thereby expected to reduce genetic variation greatly, yet recent surveys indicate that substantial variation remains in many species.  This problem has been addressed in an acoustic pyralid moth, Achroia grisella (Lesser wax moth), in which males broadcast an ultrasonic mating song attractive to females up to several meters distant.  Playback experiments show that female A. grisella prefer male songs that are distinguished by several key features, and breeding experiments demonstrate substantial genetic variance, and heritability, for these song features.  Further breeding experiments and tests with inbred lines indicate that genetic tradeoffs between life history characters and male song attractiveness are not responsible for maintaining this variance.  That is, attractive singers are larger, do not take longer to attain that size, survive longer, and spend more time singing on nightly and lifetime bases.  On the other hand, environmental variation over space and time, combined with interactions between genotypes and the environment, may contribute to the genetic variance observed for male song attractiveness:  Certain lines exhibit superior attractiveness when developing under a favorable environmental regime but show marked declines under stress, while other lines exhibit only modest performance in favorable environments but show little reduction in stress; that is, no one genetic variant exhibits the superior performance in all environments.  However, these findings have thus far been restricted to observations of highly inbred or artificially-selected laboratory populations, and the actual significance of environmental variation and genotype x environment interaction for maintaining genetic variance in natural populations is unknown. 

We are extending the above studies to determine whether 1) the range of responses to environmental gradients and 2) the environmental variation that actually occur in the field are likely to maintain the levels of genetic variance observed for male song attractiveness.  In particular, we will determine whether the level of genetic variance reflects the amount of environmental variation a population has experienced, and whether genetic variants that exhibit maximum attractiveness in both favorable and stressful environments occur. We will also determine whether variation in female preference for male signals, and the response of the female preference trait to environmental variation, contribute to genetic variance for male song attractiveness.  This possibility will be augmented via a parallel molecular genetic (quantitative trait locus, QTL) analysis of inbred lines in which they will examine the covariance between genes that influence male song and female preference traits.
Funding: NSF