Symbiont-Driven Male Mating Success in the Neotropical Drosophila paulistorum Superspecies.

Microbial symbionts are ubiquitous associates of living organisms but their role in mediating reproductive isolation (RI) remains controversial. We addressed this knowledge gap by employing the Drosophila paulistorum-Wolbachia model system. Semispecies in the D. paulistorum species complex exhibit strong RI between each other and knockdown of obligate mutualistic Wolbachia bacteria in female D. paulistorum flies triggers loss of assortative mating behavior against males carrying incompatible Wolbachia strains. Here we set out to determine whether de novo RI can be introduced by Wolbachia-knockdown in D. paulistorum males. We show that Wolbachia-knockdown D. paulistorum males (i) are rejected as mates by wild type females, (ii) express altered sexual pheromone profiles, and (iii) are devoid of the endosymbiont in pheromone producing cells. Our findings suggest that changes in Wolbachia titer and tissue tropism can induce de novo premating isolation by directly or indirectly modulating sexual behavior of their native D. paulistorum hosts.

Infectious speciation revisited: impact of symbiont-depletion on female fitness and mating behavior of Drosophila paulistorum.

The neotropical Drosophila paulistorum superspecies, consisting of at least six geographically overlapping but reproductively isolated semispecies, has been the object of extensive research since at least 1955, when it was initially trapped mid-evolution in flagrant statu nascendi. In this classic system females express strong premating isolation patterns against mates belonging to any other semispecies, and yet uncharacterized microbial reproductive tract symbionts were described triggering hybrid inviability and male sterility. Based on theoretical models and limited experimental data, prime candidates fostering symbiont-driven speciation in arthropods are intracellular bacteria belonging to the genus Wolbachia. They are maternally inherited symbionts of many arthropods capable of manipulating host reproductive biology for their own benefits. However, it is an ongoing debate as to whether or not reproductive symbionts are capable of driving host speciation in nature and if so, to what extent. Here we have reevaluated this classic case of infectious speciation by means of present day molecular approaches and artificial symbiont depletion experiments. We have isolated the α-proteobacteria Wolbachia as the maternally transmitted core endosymbionts of all D. paulistorum semispecies that have coevolved towards obligate mutualism with their respective native hosts. In hybrids, however, these mutualists transform into pathogens by overreplication causing embryonic inviability and male sterility. We show that experimental reduction in native Wolbachia titer causes alterations in sex ratio, fecundity, and mate discrimination. Our results indicate that formerly designated Mycoplasma-like organisms are most likely Wolbachia that have evolved by becoming essential mutualistic symbionts in their respective natural hosts; they have the potential to trigger pre- and postmating isolation. Furthermore, in light of our new findings, we revisit the concept of infectious speciation and discuss potential mechanisms that can restrict or promote symbiont-induced speciation at post- and prezygotic levels in nature and under artificial laboratory conditions.

Male-specific cuticular compounds of the six Drosophila paulistorum semispecies: structural identification and mating effect.

We have identified cuticular pheromones that sustain the integrity of the six Drosophila paulistorum semispecies. Hexane extracts of male and female cuticles were separated on a silica gel column and analyzed using gas chromatography/mass spectrometry. Both sexes of each of the six semispecies have the same fifteen major cuticular components, all hydrocarbons ranging from C31 to C37. However, all males have four additional ester compounds. Bioassay observations showed that this four-component ester complex imparts a strong anti-aphrodisiac effect on intra-semispecific mating behavior, thus confirming its pheromonal role. The three major ester components are methyl (Z)-9-tetradecenoate (C(15)H(28)O(2)), 11-docosenyl acetate (C(24)H(46)O(2)), and 19-triacontenyl acetate (C(32)H(62)O(2)). The fourth ester is a di-unsaturated acetate with molecular formula C(32)H(60)O(2), but the positions of unsaturation have not been determined. Bioassays indicate that the male-specific complex of the transitional semispecies, the relict ancestor, imparts anti-aphrodisiac effects on the other semispecies as well, but effectiveness decreases with phylogenetic distances. Across the six semispecies, the male-specific compounds are the same, but vary quantitatively. Apparently, the quantitative differences among these incipient species act efficiently to preclude hybridization in nature. Because Drosophila paulistorum is a cluster of incipient species, this opportunity to observe pheromonal influences on speciation is unique.

Human handedness and scalp hair whorl direction: no evidence for a common cause.

A much wider variation in many physical and cognitive behaviours exists among left-handers than among right-handers, which is evidence for more than one aetiology of left-handedness. Despite these findings some researchers are still attempting to link left-handedness to a single cause, most recently genetic determination, by pairing left-handedness with a presumed genetically based characteristic. One of these characteristics is scalp hair whorl, which may be clockwise, counter-clockwise (purportedly associated with left-handedness), or a reversal whorl (rare). Results of recent research in this topic are reviewed, our own findings are presented, and we conclude that there is no logical reason for, nor evidence of, a common aetiology of both hair whorl direction and handedness.

On the other hand.

This paper presents a brief overview of the past and current state of handedness research illustrating some of the controversies. It emphasizes two aspects: the lack of agreement on the behavior that indicates to which hand-use group (left or right) an individual belongs, and the reasons for preferring one hand rather than the other for various manual activities.

Developmental isolation and subsequent adult behavior of Drosophila paulistorum. VI. Quantitative variation in cuticular hydrocarbons.

Our previous studies have demonstrated that long-term chemical contact with heterospecifics during development minimized unproductive heterospecific matings among the six Drosophila paulistorum semispecies. When socially isolated from conspecifics, discrimination significantly decreased so that more ultimately unproductive heterospecific matings occurred. Such results suggest that learning and social experiences play roles in mate recognition, using chemical information. In investigations into the development of discriminatory behavior in D. paulistorum, social experiences influence the production of cuticular hydrocarbons in both sexes among the six different semispecies of D. paulistorum. Produced by both sexes, 2-methyl triacontane has been detected from egg stages on, gradually increasing with age. However, 11-docosenyl acetate, a male pheromone, was not produced until early adult stages. There were consistent significant differences in quantities and ratios of each of these hydrocarbons among the six semispecies, contributing to their reproductive isolation. Furthermore, we found significant quantitative differences in hydrocarbons between socially isolated flies and communally raised ones: Socially isolated flies produced significantly greater quantities of hydrocarbons than communally raised flies, consistent with previous behavioral data, as isolated flies court more vigorously and often.