Deletions in the desaturase2 gene in Israeli populations of Drosophila melanogaster: possible role in mate recognition.

Molecular analysis of the desaturase2 (desat2) gene responsible for female cuticular hydrocarbon synthesis, revealed that all tested Israeli populations of Drosophila melanogaster belong to the 'Low' type (low ratio of 5,9-/7,11-heptacosadiene) with a 16 bp deletion in the promoter region. Additional deletion leading to the appearance of the stop-codon TGA was found in exon 1 in populations inhabiting opposite slopes at Nahal Oren canyon. Mating experiments between carriers of various desat2 alleles estimated the putative effect of desat2 status on courtship behaviour. The allele with an additional deletion in exon 1 seems to play some role in mating success, which is associated with shorter courtship latency and courtship duration. The appearance and maintenance of this mutant allele in populations inhabiting Nahal Oren canyon may reflect flies' adaptation to peculiar microscale climatic conditions and may be associated with incipient sympatric differentiation.

Adaptive differentiation of thermotolerance in Drosophila along a microclimatic gradient.

We examined whether a remarkable occurrence - the physiological evolution of two Drosophila melanogaster populations, despite a spatial separation of only 100-400 m, was idiosyncratic and temporary, or persisted over multiple years. We ascertained the high-temperature tolerance of Drosophila descended from populations on the north-facing slope (NFS) and south-facing slope (SFS) of 'Evolution Canyon' (Lower Nahal Oren, Mt Carmel, Israel), which were collected in 1997, 1999, and 2000. Results for these Drosophila uniformly resembled other studies in many respects: an inverse relationship between survival and heat-shock temperature, male-female differences in thermotolerance, and inducible thermotolerance. Importantly, for all years of collection, SFS flies consistently exceeded NFS flies in basal and inducible thermotolerance after diverse heat shocks, with and without thermal pretreatment, and whether isofemale lines, synthetic populations, or inbred lines were compared. Inbred lines, however, had lower thermotolerance than outbred lines. Several nonexclusive processes may explain the evolution of such physiological differentiation.

Sequence polymorphism of candidate behavioural genes in Drosophila melanogaster flies from 'Evolution canyon'.

This study focuses on the molecular features of three candidate behavioural genes in Drosophila melanogaster from the opposite slopes of Nahal Oren Canyon, Mount Carmel, Israel. These slopes display dramatic physical and biotic contrasts. Spatial variation of microclimatic conditions leads to adaptive differentiation and partial sexual isolation of populations, as suggested by our previous studies. The chosen candidate genes presumably contributing to genetic variation in sexual behaviour of Drosophila in the Canyon were desaturase, period, and no-on-transient A. These genes are known to include polymorphic repeated sequences, insertions/deletions, or nucleotide substitutions. The idea was that their polymorphism might be one of the determinants of behavioural peculiarities of flies derived from the opposite slopes. Indeed, interslope differences in the sequence encoding the (Thr-Gly)n repeat (exon 5) of the period gene were established, suggesting evolutionary functional importance. In particular, we unraveled variation in the length and composition of this region in different NFS (north-facing slope) and SFS (south-facing slope) lines. The 'European' allele (n = 20) was a 2.6-fold more abundant on the NFS compared to the SFS. This predominance probably gives some advantages for flies inhabiting wet and less warm conditions of the NFS. We suggest that repeat length/composition may influence the functional features of flies, i.e. habitat choice, nonrandom mating, and temperature adaptation. A series of female single-mate-choice tests show that females derived from NFS distinguish between males with specific per alleles (n = 17 vs. n = 20), as well as between males originated from the opposing slopes. Females from SFS were less discriminating and did not manifest significant deviation from random mating.

Variable assortative mating in replicate mating trials using Drosophila melanogaster populations derived from contrasting opposing slopes of 'Evolution Canyon', Israel.

Significant assortative mating in laboratory studies has been previously shown between two populations of Drosophila melanogaster collected from micro-climactically contrasting and opposing slopes of 'Evolution Canyon' (Lower Nahal Oren, Israel; Korol et al., 2000). Coupled with evidence that the two populations are adapted to their respective environments, this has been suggested as a rare example of ongoing behaviourally mediated speciation occurring in the face of gene flow. Reproductive isolation between these populations, however, has never been confirmed by replicate experiments in an independent laboratory. For this reason, we tested recent collections of these populations for premating isolation in both the original (Haifa) and a new (Burnaby) laboratory under a variety of experimental protocols. Although non-random mating was found in the majority of trials conducted in Haifa, we were unable to replicate these strong results in Burnaby. Most notably, we failed to detect assortative mating in four separate double choice experiments. Significant non-random mating was detected, however, in three of six single choice experiments in Burnaby, suggesting that the populations are behaviourally differentiated in some manner. Why nonrandom mating was weaker in Burnaby than Haifa is not understood, but suggests that assortative mating may be sensitive to unknown environmental factors.

Sexual and reproductive behaviour of Drosophila melanogaster from a microclimatically interslope differentiated population of "Evolution Canyon" (Mount Carmel, Israel).

The strong microscale interslope environmental differences in "Evolution Canyon" provide an excellent natural model for sympatric speciation. Our previous studies revealed significant slope-specific differences for a fitness complex of Drosophila. This complex involved either adaptation traits (tolerance to high temperature, different viability and longevity pattern) or behavioural differentiation, manifested in habitat choice and non-random mating. This remarkable differentiation has evolved despite a very small interslope distance (a few hundred metres only). Our hypothesis is that strong interslope microclimatic contrast caused differential selection for fitness-related traits accompanied by behavioural differentiation and reinforced by some sexual isolation, which started incipient speciation. Here we describe the results of a systematic analysis of sexual behaviour in a non-choice situation and several reproductive parameters of D. melanogaster populations from the opposite slopes of "Evolution Canyon". The evidence indicates that: (i) mate choice derives from differences in mating propensity and discrimination; (ii) females from the milder north-facing slope discriminate strongly against males of the opposite slope; (iii) both sexes of the south-facing slope display distinct reproductive and behavioural patterns with females showing increased fecundity, shorter time before remating and relatively higher receptivity, and males showing higher mating propensity. These patterns represent adaptive life strategies contributing to higher fitness.

Nonrandom mating in Drosophila melanogaster laboratory populations derived from closely adjacent ecologically contrasting slopes at "Evolution Canyon".

Ecological differentiation of natural populations of Drosophila melanogaster, Drosophila simulans, and another drosophilid, Zaprionus tuberculatus, in "Evolution Canyon," Mount Carmel, Israel, is well established. The fitness complex of D. melanogaster includes oviposition temperature preferences, tolerance to high temperature, drought stress and starvation, and different longevity patterns. This remarkable differentiation has evolved despite small interslope distances (only 100-400 m), within easy dispersal distance. The differences between populations are those expected from genetic adaptation to local microclimates. How such differentiation could evolve and be maintained despite the likelihood of genetic exchange between populations is a challenging question. We hypothesized that interslope microclimatic differences caused strong differential selection for stress tolerance, accompanied by behavioral differentiation (habitat choice and reduced migration rate), reinforced by sexual isolation. Here we report highly significant mate choice by flies from different slopes of the canyon, with preference for sexual partners originating from the same slope. No preferences were found when the sexual partners belonged to different isofemale lines from the same slope.

A complex adaptive syndrome in Drosophila caused by microclimatic contrasts.

Temperature and humidity are among the most important environmental factors affecting insect adaptive strategies and evolution. Here, we report multiple adaptive differences between Drosophila melanogaster isofemale lines derived from the opposite slopes of Lower Nahal Oren canyon at Mount Carmel, Israel. The slopes are separated by 100 m at the bottom and 400 m at the top, and contrast sharply in physical and biotic factors. The multivariate fitness complex analysed in D. melanogaster included oviposition temperature preferences, viability and longevity changes, caused by short-term and lifetime temperature treatments, and resistance to drought stress at different temperatures. Some of these measures were obtained for the sibling species, D. simulans, and gave results that paralleled those of D. melanogaster. We conclude that strong microclimatic natural selection overrides migration in Drosophila at this microsite.