Cellular Basis and Developmental Timing in a Size Cline of Drosophila Melanogaster

We examined 20 Drosophila melanogaster populations collected from a 2600-km north-south transect in Australia. In laboratory culture at constant temperature and standard larval density, a genetic cline in thorax length and wing area was found, with both traits increasing with latitude. The cline in wing area was based on clines in both cell size and cell number, but was primarily determined by changes in cell number. Body size and larval development time were not associated among populations. We discuss our results in the context of selection processes operating in natural and experimental populations.     

Clinal variation in morphology and assortative mating in the soldier beetle, Chauliognathus pennsylvanicus (Goleoptera: Cantharidae)

In north Georgia populations of the soldier beetle, Chauliognathus pennsylvanicus , the length of the elytral spot varies clinally. At the southern end of a 200 km cline the distribution of spot length is unimodal with longer spot lengths predominating while at the northern end of the cline the distribution is bimodal but with shorter spot lengths being more frequent. North of the cline only short elytral spot lengths are observed, while the converse is true south of the cline. The strength of assortative mating on the basis of elytral spot length increases from south to north along the cline, resulting in complete pre-mating isolation between short and long spot length morphs at the north end of the cline. Laboratory mate choice tests indicate that assortative mating in the field is not the result of differential timing of activity or microhabitat choice but rather that it represents a real behavioural preference. Individuals from monomorphic populations on either side of the cline do not mate assortatively in the laboratory, indicating that reproductive isolation has evolved on the cline.     

Gammaglobulin allotypes of Melanesians from Malaita and Bougainville, Solomon Islands

Data are presented on the distribution of the Gm and Inv allotypes of human IgG in samples from Melanesian populations, three from Malaita and three from Bougainville of the Solomon Islands. The Lau from Malaita are polymorphic for the phenogroup, Gm^{1, 2, 5, 13, 14}. This phenogroup is not known to be polymorphic in any other population of the world. The Inv¹ frequencies of the populations from Malaita are lower than the lowest observed in samples from Bougainville, and this may indicate an extension of the north-south cline for Inv¹ previously reported for Bougainville. Samples from Aita in the north of Bougainville and from the Nagovisi in the south confirm the existence of the north-south cline for Inv¹ in Bougainville and suggest the presence of a Gm cline.     

QTL analysis of the photoperiodic response and clinal distribution of period alleles in Nasonia vitripennis

In seasonal environments, organisms synchronize their life cycle with the annual cycle of environmental factors. In many insect species, this includes a diapause response: a timed dormant stage that allows to survive harsh winter conditions. Previously, we have shown that larval diapause in the parasitic wasp Nasonia vitripennis is induced by the mother upon exposure to a threshold number of short photoperiods (named switch point) and diapause response follows a latitudinal cline in natural populations. Here, we present a QTL analysis using two lines derived from the extremes of this clinal distribution: a northern line from Oulu, Finland and a southern line from Corsica, France. A genomic region on chromosome 1 and one on chromosome 5 were found to be associated with photoperiodic diapause induction. Interestingly, these regions contain the putative clock genes period, cycle (chromosome 1) and cryptochrome (chromosome 5). An analysis of period polymorphisms in seven European populations showed a clinal distribution of two main haplotypes that correlate with the latitudinal cline for diapause induction.     

Some two locus models for the evolution of sex-determining mechanisms

Since 1958 several wild laboratory populations of the housefly (Musca domestica) have been found which have single locus “autosomal” sex determination instead of the “standard” chromosomal mechanism. In western European populations of this species there is a north-south cline with respect to sex-determining mechanisms, from the XY-XX to the completely autosomal. In addition there is a similar cline with respect to altitude a.s.1. in Italy (Franco et al., 1982). This has stimulated a theoretical study of the action of selective forces on these polymorphisms. A series of two locus models have been considered. Assuming that the populations were originally all standard, we have attempted to construct possible scenarios for the establishment of this cline. In the absence of selection it is highly unlikely that such a cline would be established. On the other hand, in the presence of either meiotic drive or viability selection on genotypes, a population can shift from one sex-determining system to another, e.g., from male heterogamety to female heterogamety and to a two locus system. The sex ratio at some of the equilibria of the systems can be different from 1:1. The results are discussed with respect to the establishment of the above cline.     

Electrophoretic variation of adductor muscle protein and tetrazolium oxidase in the smooth Washington clam, Saxidomus giganteus (Deshayes 1839)

Two electrophoretic polymorphisms (muscle protein 2 alleles and tetrazolium oxidase 3 alleles) were found in Saxidomus giganteus. The muscle protein system showed significant differences between collections from different locations, while the tetrazolium oxidase system showed variation in allele frequency suggestive of a north-south cline.     

Latitudinal cline in segment number in an arthropod species, Strigamia maritima

Arthropods vary more than 30-fold in segment number. The evolutionary origins of differences in segment number among species must ultimately lie in intraspecific variation. Yet paradoxically, in most groups of arthropods, the number of segments is fixed for each species and shows no intra- or interpopulation variation at all. Geophilomorph centipedes are an exception to this general rule, and exhibit intraspecific variation in segment number, with differences between individuals being determined during embryonic development and hence independent of population age structure. Significant differences in segment number between different geographical populations of the same species have been previously reported, but insufficient sampling has been conducted to reveal any particular geographical pattern. Here, we reveal a latitudinal cline in segment number in the geophilomorph species Strigamia maritima: segment number in British populations decreases with distance north. This is the first such cline to be reported for any centipede species; indeed as far as we are aware it is the first such cline reported for any arthropod species. In vertebrates, fish are known to exhibit a latitudinal cline in segment number, but interestingly, this is in the opposite direction; fish add segments with increasing latitude, centipedes subtract them.     

Starvation resistance and adult body composition in a latitudinal cline of Drosophila melanogaster

Abstract Latitudinal geographic variation in Drosophila melanogaster is pervasive. Parallel clines in traits such as body size, egg size, ovariole number, and development time have been found on several continents throughout the world. However, a cline in starvation resistance and fat content in D. melanogaster has so far been found only in India. Here we investigate starvation resistance and fat content in 10 populations from South America, in which clines in body size, egg size, and development time have previously been found. We find no evidence for a cline in starvation resistance or fat content in South America. We therefore suggest that the cline in starvation resistance in India may have evolved in response to specific climatic variation found only in India.     

Meta-analysis of GJB2 mutation 35delG frequencies in Europe

Mutations in the gene encoding connexin-26 (specified GJB2) have been shown to be a major cause of nonsyndromic recessive deafness (NSRD), and a single mutation 35delG in the GJB2 gene accounts for the majority of cases of NSRD. This mutation was screened in France and in other European populations by a reliable PCR method. We present here a meta-analysis of the 35delG frequencies in 4123 random controls from 20 European countries, and show that the mutation is more frequent in the south of Europe than in the north; a north-south increasing cline of 35delG frequencies is established (r = -0.527).     

Genetic variation of esterase D (EsD) polymorphism in Indian populations

Data are presented on the phenotypes and gene frequencies of esterase D (EsD) polymorphism in various endogamous caste groups of Patiala and Faridkot districts of Punjab, north-west India. The frequency of the EsD2 allele in these groups varies from a minimum of 0.157 in Khatris to a maximum of 0.253 in Ramdasia Sikhs, a range quite typical of the north Indian populations. Examination of the available data on esterase D system in indigenous populations demonstrates the existence of a north-south cline in the distribution of the EsD2 allele in India.     

Divergent tail and throat ornamentation in the barn swallow across the Japanese islands

The geographic variations in male ornamentation provide insights into how different populations reach a different mean trait value under opposing forces of natural and sexual selection. Although the latitudinal cline of the elongated tail streamer, a sexually selected trait in the European subspecies of the barn swallow Hirundo rustica rustica, is a classic example, it has recently been shown that other subspecies of swallows have different targets of sexual selection. Here, we studied the latitudinal cline of ornamentation in the Asian subspecies, H. r. gutturalis, in which not the tail length but the white tail spot and red throat patch are important sexually selected traits. After controlling for covariates, the size of the white tail spot increased with latitude, while the size of the red throat patch decreased with latitude. On the other hand, we could not find any clear pattern regarding the elongated tail streamer, measured as fork depth. The divergent ornamentation across populations could be explained by latitudinal clines of sexually selected advantages of each ornament.     

Population structure in Mediterranean islands and risk of genetic invasion in Culexpipiens L. (Diptera: Culicidae)

The mosquito Culex pipiens is subjected to organophosphate treatments in both Corsica and southern Sardinia, but the resistance gene A2-B2, which is currently in a worldwide expansion, has only reached Sardinia. In order to understand this situation, the genetic structure of populations sampled in Sardinia and Corsica was assessed using 15 isozymes. Two loci (HK1 and HK2) were not taken into account because of the possibility of selection. For trie other loci, statistical independence was not rejected for all possible pairs, and no deviation from Hardy-Weinberg expectations was apparent. Low but significant genie differentiation was present between Corsica and Sardinia, as well as between northern and southern Sardinia, despite a large number of effective migrants per generation. These results are discussed in the context of the high probability of extinction/recolonization of breeding sites, the flight migration ability of this mosquito, and the pleiotropic cost of insecticide resistances genes. It is concluded that A2-B2 resistance is unlikely to reach Corsica from southern Sardinia, unless accidental human transportation occurs.     

Normal and aberrant Mesocestoides tetrathyridia from Crocidura spp. (Soricimorpha) in Corsica and Spain

Tetrathyridia of Mesocestoides sp. were collected from the body cavities of the shrews (Insectivora), Crocidura russula, in Valencia, Spain and Crocidura suaveolens on the Mediterranean island of Corsica, France. Specimens were processed by routine microscopic and histological techniques, including examination with brightfield, phase-contrast, and differential-interference-contrast optics. Most tetrathyridia were clustered together inside host-derived fibrotic capsules, but some occurred free in the body cavity. All specimens examined from both locations had solid hindbodies, i.e., lacking a primary lacuna, thus conforming to the plerocercoid metacestode type; all possessed a single normal tetra-acetabulate scolex. All metacestodes from C. russula in Valencia were normal tetrathyridia. Those from C. suaveolens in Corsica were either normal tetrathyridia or had aberrant deep convolutions of an unusually elongated hindbody. No tetrathyridium from either location or host showed tegumental or excretory duct anomalies such as those reported by several authors from aberrant tetrathyridia and spargana in some other locations. No definitive evidence of asexual proliferation was visible in any of the tetrathyridia, but those with abnormally convoluted hindbodies from a single C. suaveolens in Corsica suggest the potential for asexuality by fission of the hindbody. These results add to our understanding of morphological and developmental variation among metacestodes in this widespread and variable genus.     

How does arthropod segment number evolve?—some clues from centipedes

Summary Studies of intraspecific variation in the number of trunk segments of geophilomorph centipedes provide clues as to how different species of arthropods, and whole clades in some cases, come to be characterized by different segment numbers. However, although previous work in this area has revealed an interesting geographical pattern—a latitudinal cline in which segment number decreases with increasing latitude—the causality of the cline remains obscure. Is it because of selection on genetically based variation, or is it a result of a form of phenotypic plasticity in which the segmentation process is directly affected by a latitude‐correlated factor such as temperature? Here, we provide some indirect evidence for plasticity. If the cline is indeed a plastic one, a paradox arises, because the cline mirrors interspecific variation—geophilomorph species with more northern ranges typically have fewer segments than those from further south—but interspecific differences cannot arise from nonheritable variation. We propose a resolution of this apparent paradox via a model in which genetic and environmental factors interact through selection acting on developmental reaction norms.     

Evolution of insecticide resistance in Culex pipiens populations: The Corsican paradox

Culex pipiens mosquitoes from Corsica have been subject to insecticide treatments since 1971, using temephos (an organophosphate). After 17 years, resistance has not developed beyond a 14-fold level. This relatively low resistance is due to the presence of several identified resistance genes, including the insensitive target ( Ace^R ) and overproduced esterases (A1, A4 and B4). The fact that only a low resistance has developed after 17 years of treatment and that this low resistance level is the result of a relatively large number of resistance genes constitute a paradox. To understand this situation and explain why a higher temephos resistance level has not evolved in Corsica as in other parts of the world, it is proposed that the occurrence (through mutation or migration) of efficient resistance genes was a limiting step, and that the only resistance genes available at that time through migration from the surrounding Mediterranean countries had a low cross-resistance to temephos. The local situation of Corsica is discussed in the light of recent data on the world distribution of the known organophosphate resistance genes in this species, and the relative role of mutation and migration in the evolution of insecticide resistance in natural populations.     

Quantitative flavonoid variation in Lupinus sericeus

One hundred and eighty-one plants representing 32 individual populations of Lupinus sericeus were collected from throughout the range of the species. Quantitative analyses of flavonoid derivatives were performed using high performance liquid chromatography. Peak height data were subjected to multivariate analysis. Principal components analysis suggested the existence of a north-south cline based upon differences in orientin concentration. Canonical variates analysis suggested that southerly, central and northerly populations were distinguishable. Simple statistical tests on orientin concentration, the major flavonoid seen in the study, clearly distinguished southerly populations from all others. Seeds were collected from representative populations from the range of L. sericeus and the plants obtained therefrom were grown under uniform conditions. Analysis of orientin concentration in these plants showed no significant differences between populations. We conclude from these studies that all L. sericeus plants appear to have similar capacities to accumulate orientin, and that differences in concentration of this compound seen in naturally occurring plants are a reflection of environmental influences and cannot be used as support for any infrageneric taxonomic distinctions.     

Phylogeography and founder effect of the endangered Corsican red deer (<Emphasis Type="Italic">Cervus elaphus corsicanus</Emphasis>)

Red deer (n = 149) from eight geographical locations, including the endangered endemic populations from the Tyrrhenian islands (Sardinia and Corsica), were analysed at eight polymorphic microsatellite loci. Two questions were addressed: (1) Is there a founder effect in the Corsican population, which was reintroduced to the island using Sardinian deer after the species’ extinction on Corsica? (2) What is the origin of the Tyrrhenian or Corsican red deer (Cervus elaphus corsicanus)? Our results showed signs of a founder effect for the red deer on Corsica in that these deer showed differentiation from the Sardinian population as measured by F_ST values, assignment tests (with and without a priori definition of populations) and individual-based dendrograms. Genetic variability, however, did not differ significantly between the two populations. With respect to the phylogeography of C. e. corsicanus we found that both deer from North-Africa and Mesola on the Italian mainland were genetically close to the Corsican red deer, but phylogenetic trees based on genetic distances were only poorly supported statistically. Among all populations studied the Mesola red deer showed the lowest distance values from Corsican red deer and yielded allele frequencies that were more similar to those of C. e. corsicanus than were those of North-African red deer. These results are in line with recent palaeontological and archaeozoological findings which suggest that the Corsican red deer is derived from small Italian red deer introduced from the mainland to Sardinia and Corsica during the Late Neolithic and just before the beginning of Classical Antiquity, respectively. They also suggest a possible recent introduction of Tyrrhenian red deer to North-Africa (rather than the other way around), thus accounting for the close genetic relationship (especially based on mitochondrial DNA) that has repeatedly been found between C. e. corsicanus and C. e. barbarus.     

Mitochondrial ghost lineages blur phylogeography and taxonomy of Natrix helvetica and N. natrix in Italy and Corsica

Grass snakes are widely distributed across the Western Palearctic. Recent phylogeographic studies provided evidence that three distinct parapatric species exist. Two of these occur in Italy, Natrix helvetica and N. natrix, and a contact zone between both taxa has been suggested for north-eastern Italy. Moreover, previous investigations revealed for the Italian Peninsula a complex phylogeographic structure. Using mtDNA sequences and microsatellite loci, we examined the situation for mainland Italy, Sicily, Sardinia, and Corsica. Our study confirmed the occurrence of N. natrix in north-eastern Italy. Cline analyses revealed limited gene flow between N. helvetica and N. natrix across a narrow hybrid zone. Within N. helvetica, conflicting patterns of mitochondrial and nuclear genomic differentiation were revealed. Three nuclear genomic clusters were found; one of them corresponded to no fewer than five distinct and, in part, deeply divergent and ancient mitochondrial lineages from mainland Italy and Sicily. This cluster was paraphyletic with respect to the two remaining mitochondrial lineages, each of which matched with another nuclear genomic cluster (one from Corsica plus Sardinia and another one from western Europe north of the Alps). This unexpected pattern most likely results from mainly male-mediated gene flow and female philopatry combined with population-density-dependent processes such as ‘high-density blocking’. With respect to taxonomy, we propose to synonymize N. h. lanzai Kramer, 1970 with N. h. sicula (Cuvier, 1829), acknowledging their lacking nuclear genomic differentiation. The studied hybrid zone of N. h. helvetica and N. h. sicula in Italy is wide, with a smooth cline for nuclear markers, supporting their subspecies status. We found no evidence for the distinctiveness of the two subspecies from Corsica (N. h. corsa) and Sardinia (N. h. cetti), suggesting their synonymy, but refrain from taxonomic conclusions because of small sample sizes and the endangered status of the Sardinian taxon.