William T. Starmer

Ecological and evolutionary genetics of Drosophila.

XVIII International Congress of Entomology, Vancouver, July

Parasite–induced risk of mortality elevates reproductive effort in male Drosophila

A trade–off between sex and somatic maintenance is fundamental to life–history theory. Tests of this trade–off usually emphasize deleterious consequences of increased reproduction on life span. Here we show the reverse effect, that reductions in the expected life span elevate sexual activity. Experimentally parasitized male Drosophila nigrospiracula lived shorter lives, but before dying, they courted females significantly more than unparasitized controls. This greater courtship resulted in increased mating speed, and potentially greater reproductive success than parasitized males would have achieved otherwise. The results show that an environmental reduction in life span increases reproductive effort, and support the hypothesis of a trade–off between these key life–history traits.

Ecology and yeasts

Publisher Summary This chapter focuses on yeast ecology in general, and its possible future developments. Yeasts do not occur randomly throughout the biosphere. They form communities of species. Each community may be defined by its habitat, which is the actual place where an assemblage of yeasts lives, and by the niches of its component species. The niche consists of the attributes that make yeast capable of sharing a habitat with other members of the community. Thus, the niche is the sum of all physical, chemical, or biotic factors required for successful existence. The members of a community may have different niches that happen to overlap in a given habitat. By their very nature, yeasts are generally limited in the range of habitats they can occupy. In addition to various mineral nutrients that serve as building blocks for most of their cell constituents, yeasts require significant amounts of an organic source of carbon and energy of relatively small molecular weight. The high surface/volume ratio of yeasts favors rapid nutrient absorption. Their unicellular nature often makes them better suited for deep liquid substrates or moist and uneven surfaces, unlike molds, which form hyphae that can penetrate and liquefy semisolid substrates or spread over smooth, inert surfaces. Yeasts usually grow over a broad range of pH values, allowing them in particular to colonize materials that have already been the site of fermentative activities by bacteria. Yeasts also fulfill an important role in the food chain. Insects not only feed on substrates known to serve as yeast habitats, but also exhibit several adaptations to yeast utilization as a feedstock.

Coadaptation ofDrosophila and yeasts in their natural habitat

The mutualistic interactions of cactophilicDrosophila and their associated yeasts in the Sonoran Desert are studied as a system which has evolved within the framework of their host cactus stem chemistry. Because theDrosophila-yeast system is saphrophytic, their responses are not thought to directly influence the evolution of the host. Host cactus stem chemistry appears to play an important role in determining where cactophilicDrosophila breed and feed. Several chemicals have been identified as being important. These include sterols and alkaloids of senita as well as fatty acids and sterol diols of agria and organpipe cactus. Cactus chemistry appears to have a limited role in directly determining the distribution of cactus-specific yeasts. Those effects which are known are due to unusual lipids of organpipe cactus and triterpene glycosides of agria and organpipe cactus.Drosophilayeast interactions are viewed as mutualistic and can take the form of (1) benefits to theDrosophila by either direct nutritional gains or by detoxification of harmful chemicals produced during decay of the host stem tissue and (2) benefits to the yeast in the form of increased likelihood of transmission to new habitats. Experiments on yeast-yeast interactions in decaying agria cactus provide evidence that the yeast community is coadapted. This coadaptation among yeasts occurs in two manners: (1) mutualistic increases in growth rates (which are independent of the presence ofDrosophila larvae) and (2) stabilizing competitive interactions when growth reaches carrying capacity. This latter form is dependent on larval activity and results in benefits to the larvae present. In this sense, the coadapted yeast community is probably also coadapted with respect to itsDrosophila vector.

MECHANISMS UNDERLYING THE SPERM QUALITY ADVANTAGE IN DROSOPHILA MELANOGASTER

Abstract Contrary to early predictions of sperm competition theory, postcopulatory sexual selection favoring increased investment per sperm (e.g., sperm size, sperm quality) has been demonstrated in numerous organisms. We empirically demonstrate for Drosophila melanogaster that both sperm quality and sperm quantity independently contribute to competitive male fertilization success. In addition to these independent effects, there was a significant interaction between sperm quality and quantity that suggests an internal positive reinforcement on selection for sperm quality, with selection predicted to intensify as investment per sperm increases and the number of sperm competing declines. The mechanism underlying the sperm quality advantage is elucidated through examination of the relationship between female sperm-storage organ morphology and the differential organization of different length sperm within the organ. Our results exemplify that primary sex cells can bear secondary sexual straits.

Molecular Basis of Spectral Tuning in the Red- and Green-Sensitive (M/LWS) Pigments in Vertebrates

Vertebrate vision is mediated by five groups of visual pigments, each absorbing a specific wavelength of light between ultraviolet and red. Despite extensive mutagenesis analyses, the mechanisms by which contemporary pigments absorb variable wavelengths of light are poorly understood. We show that the molecular basis of the spectral tuning of contemporary visual pigments can be illuminated only by mutagenesis analyses using ancestral pigments. Following this new principle, we derive the “five-sites” rule that explains the absorption spectra of red and green (M/LWS) pigments that range from 510 to 560 nm. Our findings demonstrate that the evolutionary method should be used in elucidating the mechanisms of spectral tuning of four other pigment groups and, for that matter, functional differentiations of any other proteins.

An Analysis of the Yeast Flora Associated with Cactiphilic Drosophila and their Host Plants in the Sonoran Desert and Its Relation to Temperate and Tropical Associations

A survey was made in the Sonoran Desert of yeasts living in the decaying arms of five species of cereus cacti and the four species of Drosophila that utilize them as host plants. The most common yeasts among 132 isolates from the cacti and 187 isolates from the files, respectively, were: Pichia membranaefaciens (45% and 67%), Candida ingens (17% and 4%), Torulopsis sonorensis (12% and 11%), and Cryptococcus cereanus (8% and 7%). Eighty—eight percent of the 66 initial isolates of P. membranaefaciens from Drosophila pachea and its host, senita cactus, assimilated D—xylose while only 12% of the remaining 257 initial isolates did so. Nineteen of the 20 isolates of T. sonorensis from files were found in Drosophila mojavensis and 12 of the 14 isolates of Cryptococcus cereanus came from D. pachea. The highest mean number of yeast species per cactus was 2.77 plus or minus 0.68 in organpipe cactus and per fly was 1.63 plus or minus 0.53 in D. pachea. The flies usually carried fewer yeast species than were found in the host plant but D. pachea had almost the same mean and variance as its host, senita cactus, which had 1.64 @+ 0.40. Yeast species diversity and average niche overlap have the following rank order among habitats and localities: temperate trees > temperate flies > tropical flies > desert cacti > desert flies. Habitat diversity and average niche width show: tropical flies > temperate flies and desert cacti > desert flies and temperate trees. The physiological properties of the desert yeasts are most similar to those of the tropical yeast. However, desert yeasts have similarities with yeasts of temperate trees. Both have low fermentative ability and high assimilation ability of several alcohols and acids. The genus Pichia is by far the most common yeast genus associated with Drosophila in all habitats analyzed (36% of 1,426 isolates).

Complex interactions with females and rival males limit the evolution of sperm offence and defence

Postcopulatory sexual selection favours males which are strong offensive and defensive sperm competitors. As a means of identifying component traits comprising each strategy, we used an experimental evolution approach. Separate populations of Drosophila melanogaster were selected for enhanced sperm offence and defence. Despite using a large outbred population and evidence of substantive genetic variation for each strategy, neither trait responded to selection in the two replicates of this experiment. Recent work with fixed chromosome lines of D. melanogaster suggests that complex genotypic interactions between females and competing males contribute to the maintenance of this variation. To determine whether such interactions could explain our lack of response to selection on sperm offence and defence, we quantified sperm precedence across multiple sperm competition bouts using an outbred D. melanogaster population exhibiting continuous genetic variation. Both offensive and defensive sperm competitive abilities were found to be significantly repeatable only across matings involving ejaculates of the same pair of males competing within the same female. These repeatabilities decreased when the rival male stayed the same but the female changed, and they disappeared when both the rival male and the female changed. Our results are discussed with a focus on the complex nature of sperm precedence and the maintenance of genetic variation in ejaculate characteristics.

Detection of tomato mosaic tobamovirus RNA in ancient glacial ice

Abstract Tomato mosaic tobamovirus is a very stable plant virus with a wide host range, which has been detected in plants, soil, water, and clouds. Because of its stability and prevalence in the environment, we hypothesized that it might be preserved in ancient ice. We detected tomato mosaic tobamovirus RNA by reverse-transcription polymerase chain reaction amplification in glacial ice subcores <500 to approximately 140,000 years old from drill sites in Greenland. Subcores that contained multiple tomato mosaic tobamovirus genotypes suggest diverse atmospheric origins of the virus, whereas those containing tomato mosaic tobamovirus sequences nearly identical to contemporary ones suggest that recent tomato mosaic tobamovirus populations have an extended age structure. Detection of tomato mosaic tobamovirus in ice raises the possibilities that stable viruses of humans and other hosts might be preserved there, and that entrapped ancient viable viruses may be continually or intermittently released into the modern environment.

A mating plug and male mate choice inDrosophila hibisciBock

The functional significance of mating plugs and their relation to male mating behaviour in insects are obscure. In Drosophila hibisci, we describe a firm, gelatinous, mating plug that fills the entire uterus at copulation, and evaluate two nonmutually exclusive hypotheses for the plugs function: (1) retention of sperm near the openings of the sperm storage organs and (2) inhibition of further matings. Unlike full-sized plugs, smaller plugs produced by previously mated males failed to retain sperm at the anterior end of the uterus, indicating that full-sized plugs prevent sperm backflow away from female storage organs. Sexually mature males failed to copulate with previously mated, young females, suggesting that the plug may also deter rival matings. In newly emerged females, the plug remains within the uterus for up to 4 days after copulation, but in mature females, the plug and ejaculate are expelled whenever the next mature oocyte descends into the uterus from the common oviduct, which may be soon after copulation. Males remained in copula significantly longer with mature females, perhaps to compensate for this greater likelihood of ejaculate expulsion. Males showed a mating preference for young virgins over older virgin and nonvirgin females. This unusual mating preference may confer fertility benefits made available through the effectiveness of the plug in reducing sperm competition. Copyright 1998 The Association for the Study of Animal Behaviour.