The phylogeny of nine species of the Drosophila obscura group inferred by the banding homologies of chromosomal regions. III. Element D.

The phylogenetic relationships among nine species belonging to the obscura group of the genus Drosophila were deduced, based on similarities of the banding pattern of their polytene chromosomal element D. These similarities were inferred by the comparison of chromosomal photomaps. The phylogenetic reconstruction was the most parsimonious based on seriation by overlapping inversions and on the principle of conservation/disassociation of nearby located segments. The gene sequences of element D for all species studied were relatively easy to recognize in terms of the map of D. obscura, already found to occupy a relative central position in this group. Thus, three clusters of closely related species could be identified: obscura (D. obscura, D. ambigua, and D. tristis), African (D. kitumensis and D. microlabis), and subobscura (D. subobscura, D. madeirensis and D. guanche), with D. subsilvestris standing apart. The results are in agreement with those from the previously studied elements B and E, but element D was found to be much more conclusive concerning the links among the different clusters. Thus, it is inferred that D. guanche occupies an intermediate position between the other two species of its own cluster and all the others. The gene arrangement of D. obscura, directly related to those of the other species, has been identified. In the phylogenetic tree proposed, both the African cluster and D. subsilvestris derive from a hypothetical gene arrangement, intermediate in the pathway between the subobscura and obscura clusters.     

The phylogeny of nine species of the Drosophila obscura group inferred by the banding homologies of chromosomal regions. I. Element B

The phylogenetic relationships among nine Drosophila species belonging to the obscura group were investigated by establishing the segments displaying banding homologies in their element B (equivalent to the U element of D. subobscura). The phylogenetic ordering of the species was accomplished using overlapping inversions. Two African species, D. kitumensis and D. microlabis, were investigated. These species are homosequential for their element B gene arrangement but differ from that of D. obscura by several rearrangements. Drosophila obscura seems to be most closely related to D. subsilvestris, from which the respective element B gene arrangements differ at least by six inversions. Three species, D. obscura, D. ambigua, and D. tristis, are closely related and form a cluster. Drosophila obscura displays an element B polymorphism for a pericentric inversion for which D. ambigua is fixed for one gene arrangement and D. tristis for the other. Both D. ambigua and D. tristis share a short distal inversion in the small arm of the chromosome, and differ in this respect from D. obscura. Drosophila madeirensis, D. guanche, and D. subobscura all share the same element B gene arrangement, which is acrocentric, but metacentric in all the other species mentioned. It was found that the gene arrangements of the species from the obscura cluster seem to occupy an intermediate position between those of the species of the D. subobscura cluster and those of the African one. The data reported generally are in good agreement with information provided in the literature.     

Phylogeny of the Drosophila obscura group as inferred from one- and two-dimensional protein electrophoresis

The phylogenetic relationships of 15 species of the obscura group of Drosophila were analysed by use of one- and two-dimensional electrophoresis. Genetic distances based on two-dimensional data are five times smaller than those based on native proteins. From the data, it is proposed that the species radiation of the obscura group happened in two evolutionary bursts, the first one giving rise to at least four palearctic proto-lineages ( bifasciata, obscura (including D. subsilvestris ), subobscura , and microlabis ) and one or two proto-nearctic lineages ( affinis, pseudoobscura ), and the second, more recent burst giving rise to the current speciation within lineages.

Zusammenfassung

Phylogenie der Arten der Drosophila obscura-Gruppe abgeleitet von ein- und zweidimensionaler Protein-Elektrophorese
Die phylogenetischen Verwandtschaftsbeziehungen von 15 Arten der obscura -Gruppe der Gattung Drosophila wurden mit Hilfe von ein-und zweidimensionaler Elektrophorese von Proteinen untersucht. Die genetische Distanzen, die aus den Ergebnissen der zweidimensionalen Elektrophoresen ermittelt wurden, waren fünfmal kleiner als solche, die von nativen Proteinen kommen. Aufgrund der Untersuchungsergebnisse wird angenommen, daß die Radiation der Arten der obscura-Gruppe in zwei evolutiven Schüben erfolgt sei; der erste Schub hätte zu zumindest vier palaerktischen ( bifasciata, obscura mit D. subsilvestris, subobscura und microlabis ) und zwei proto-ne arktischen Linien ( affinis, pseudoobscura ) geführt. In einem zweiten Schub wären dann die endgültigen rezenten Arten entstanden.     

Evolutionary dynamics of the SGM transposon family in the Drosophila obscura species group

SGM (Drosophila subobscura, Drosophila guanche, and Drosophila madeirensis) transposons are a family of transposable elements (TEs) in Drosophila with some functional and structural similarities to miniature inverted-repeat transposable elements (MITEs). These elements were recently active in D. subobscura and D. madeirensis (1-2 MYA), but in D. guanche (3-4 MYA), they gave rise to a species-specifically amplified satellite DNA making up approximately 10% of its genome. SGM elements were already active in the common ancestor of all three species, giving rise to the A-type specific promoter section of the P:-related neogene cluster. SGM sequences are similar to elements found in other obscura group species, such as the ISY elements in D. miranda and the ISamb elements in Drosophila ambigua. SGM elements are composed of different sequence modules, and some of them, i.e., LS and LS-core, are found throughout the Drosophila and Sophophora radiation with similarity to more distantly related TEs. The LS-core module is highly enriched in the noncoding sections of the Drosophila melanogaster genome, suggesting potential regulatory host gene functions. The SGM elements can be considered as a model system elucidating the evolutionary dynamics of mobile elements in their arms race with host-directed silencing mechanisms and their evolutionary impact on the structure and composition of their respective host genomes.     

Differentiation of Muller''s Chromosomal Elements D and E in the Obscura Group of Drosophila

Twenty-two markers located on Muller's elements D or E have been mapped by in situ hybridization in six species of the obscura group of Drosophila and in D. melanogaster. The obscura species can be grouped into a Palearctic cluster (D. subobscura, D. madeirensis and D. guanche) and a Nearctic one (D. pseudoobscura, D. persimilis and D. miranda). Eleven of the probes contain known genes: E74, Acp70A, Est5, hsp28/23, hsp83, emc, hsp70, Xdh, Acph-1, Cec and rp49. The remaining probes are recombinant phages isolated from a D. subobscura genomic library. All these markers hybridize to the putative homologous chromosome or chromosomal arm of elements D and E. Thus, these elements have conserved their genic content during species divergence. Chromosomal homologies proposed previously for each element among the species of the same cluster have been compared with the present results. The distribution of markers within each element has changed considerably as inferred from pairwise comparisons of obscura species included in the two different clusters. Only chromosomal segments defined by closely linked markers have been conserved: one such segment has been detected in element D and three in element E between D. subobscura and D. pseudoobscura.     

Distribution of the bilbo non-LTR retrotransposon in Drosophilidae and its evolution in the Drosophila obscura species group

The bilbo element is a non-LTR retrotransposon isolated from Drosophila subobscura. We conducted a distribution survey by Southern blot for 52 species of the family Drosophilidae, mainly from the obscura and melanogaster groups. Most of the analyzed species bear sequences homologous to bilbo from D. subobscura. In the obscura group, species from the same species subgroup also share similar Southern blot patterns. To investigate the phylogenetic relationship among these elements, we analyzed eight copies of a short sequence of the element from several species of the obscura group. The obtained phylogram agrees with the phylogeny of the species, which suggests vertical transmission of the element.     

Adaptive significance of amylase polymorphism in Drosophila XIII. Old World obscura species subgroup divergence according to biochemical properties of alpha-amylase

Biochemical properties of enzyme alpha-amylase were surveyed in Drosophila obscura Old world group of species (D. subobscura, D. ambigua, D. obscura and D. tristis) sampled in the same habitat, with the aim to reveal some ecological and evolutionary aspects of amylase polymorphism, which has been studied extensively in D. subobscura, but not compared with other species in the group. The data obtained show that D. subobscura is distinct from the other three species regarding all biochemical amylase properties. Such a divergence also correlates with the niche breadth and relative abundance of these species in the same habitat.     

Population densities in Drosophila obscura Fallén and D. subobscura Collin

Abstract. 1. The population densities of Drosophila obscura and D. subobscura in mixed deciduous woodland were investigated by multiple recapture techniques, marking flies with micronized fluorescent dusts.
2. Approximate densities per 100 m² were as follows: September 1974: D.subobscura males 5, D.subobscura females 31. April 1975: D.obscura males 1, D.obscura females 2; D.subobscura males 0.3, D.subobscura females 1. June 1975: D.subobscura males 6, D.subobscura females 9.
3. Actual estimates have been compared using relative estimates derived from a reanalysis of Shorrocks's (1975) data.
4. Throughout most of the year D.subobscura was numerically dominant, rising to a peak of 750/100 m² in the winter; but in late spring D.obscura was dominant, having reduced winter mortality by diapausing.
5. D.obscura 's density was comparable with those of its Nearctic relatives; D.subobscura 's was considerably higher.
6. The effects of previous workers' approximations on the estimation of population parameters have been examined, and possible theoretical consequences have been indicated.     

Nucleotide polymorphism in the RpII215 gene region of the insular species Drosophila guanche: reduced efficacy of weak selection on synonymous variation

An approximately 6.9-kb region encompassing the RpII215 gene was sequenced for 24 individuals of the island endemic species Drosophila guanche. The comparative analysis of synonymous polymorphism and divergence in D. guanche and D. subobscura, two species with pronounced differences in population size, allows contrasting the nearly neutral character of synonymous mutations. In D. guanche, unlike in D. subobscura, (1) the ratio of preferred to unpreferred synonymous changes was similar for polymorphic and fixed changes, (2) the numbers of preferred and unpreferred changes, both polymorphic and fixed, could be explained by the mutational process, and (3) the estimated scaled selection coefficient for unpreferred mutations did not differ significantly from zero. Additionally, the comparative analysis revealed that both the ratio of preferred to unpreferred synonymous changes and the frequency spectrum of unpreferred polymorphic mutations differed significantly between species. All these results indicate that a large fraction of synonymous mutations in the RpII215 gene behave as effectively neutral in D. guanche, whereas they are weakly selected in D. subobscura. The reduced efficacy of selection in the insular species constitutes strong evidence of the nearly neutral character of synonymous mutations and, therefore, of the role of weak selection in maintaining codon bias.     

Molecular Organization of the X Chromosome in Different Species of the Obscura Group of Drosophila

Nine single copy regions located on the X chromosome have been mapped by in situ hybridization in six species of the obscura group of Drosophila. Three Palearctic species, D. subobscura, D. madeirensis and D. guanche, and three Nearctic species, D. pseudoobscura, D. persimilis and D. miranda, have been studied. Eight of the regions include known genes from D. melanogaster (Pgd, zeste, white, cut, vermilion, RNA polymerase II 215, forked and suppressor of forked) and the ninth region (lambda DsubF6) has not yet been characterized. In all six species, as in D. melanogaster, all probes hybridize to a single site. Established chromosomal arm homologies of Muller's element A are only partly supported by present results since two of the probes (Pgd and zeste) hybridize at the proximal end of the XR chromosomal arm in the three Nearctic species. In addition to the centric fusion of Muller's A (= XL) and D (= XR) elements, the metacentric X chromosome of the Nearctic species requires a pericentric inversion to account for this result. Previously proposed homologies of particular chromosomal regions of the A (= X) chromosome in the three species of the D. subobscura cluster and of the XL chromosomal arm in the three species of the D. pseudoobscura cluster are discussed in light of the present results. Location of the studied markers has changed drastically not only since the divergence between the melanogaster and obscura groups but also since the Palearctic and Nearctic species of the obscura group diverged.     

Synonymous rates at the RpII215 gene of Drosophila: variation among species and across the coding region

The region encompassing the RpII215 gene that encodes the largest component of the RNA polymerase II complex (1889 amino acids) has been sequenced in Drosophila subobscura, D. madeirensis, D. guanche, and D. pseudoobscura. Nonsynonymous divergence estimates (Ka) indicate that this gene has a very low rate of amino acid replacements. Given its low Ka and constitutive expression, synonymous substitution rates are, however, unexpectedly high. Sequence comparisons have allowed the molecular clock hypothesis to be tested. D. guanche is an insular species and it is therefore expected to have a reduced effective size relative to D. subobscura. The significantly higher rate of synonymous substitutions detected in the D. guanche lineage could be explained if synonymous mutations behave as nearly neutral. Significant departure from the molecular clock hypothesis for synonymous and nonsynonymous substitutions was detected when comparing the D. subobscura, D. pseudoobscura, and D. melanogaster lineages. Codon bias and synonymous divergence between D. subobscura and D. melanogaster were negatively correlated across the RpII215 coding region, which indicates that selection coefficients for synonymous mutations vary across the gene. The C-terminal domain (CTD) of the RpII215 protein is structurally and functionally differentiated from the rest of the protein. Synonymous substitution rates were significantly different in both regions, which strongly indicates that synonymous mutations in the CTD and in the non-CTD regions are under detectably different selection coefficients.     

The actin loci in the genus Drosophila: establishment of chromosomal homologies among five palearctic species of the Drosophila obscura group by in situ hybridization

Chromosomal homologies among the four palearctic Drosophila obscura group species D. ambigua, D. tristis, D. obscura, and D. subsilvestris and the "trans-palearctic" species D. bifasciata were established by in situ hybridization using the 5C actin gene of D. melanogaster as a probe. In all species two labeling sites were detected in each of chromosomal elements C and E and one in each of chromosomal elements A and D. In addition one labeling site was detected on element B for the species D. subsilvestris and D. bifasciata. The conservative distribution pattern of the genes of the actin multigene family, the similarities of the locations of the actin genes in the chromosomes of the five species studied, together with the concordant evidence of synteny of visible and other genetic markers as well as the similarities in banding patterns, all agree with the conclusion that the chromosomal elements have retained their essential identity throughout the evolution of these species. Using in situ hybridization detailed information of some homologous regions of chromosomes can also be established.     

Changes in the recombinational environment affect divergence in the yellow gene of Drosophila

The complete coding region of the yellow (y) gene was sequenced in different Drosophila species. In the species of the melanogaster subgroup (D. melanogaster, D. simulans, D. mauritiana, D. yakuba, and D. erecta), this gene is located at the tip of the X chromosome in a region with a strong reduction in recombination rate. In contrast, in D. ananassae (included in the ananassae subgroup of the melanogaster group) and in the obscura group species (D. subobscura, D. madeirensis, D. guanche, and D. pseudoobscura), the y gene is located in regions with normal recombination rates. As predicted by the hitchhiking and background selection models, this change in the recombinational environment affected synonymous divergence in the y-gene-coding region. Estimates of the number of synonymous substitutions per site were much lower between the obscura group species and D. ananassae than between the species of the obscura group and the melanogaster subgroup. In fact, a highly significant increase in the rate of synonymous substitution was detected in all lineages leading to the species of the melanogaster subgroup relative to the D. ananassae lineage. This increase can be explained by a higher fixation rate of mutations from preferred to unpreferred codons (slightly deleterious mutations). The lower codon bias detected in all species of the melanogaster subgroup relative to D. ananassae (or to the obscura group species) would be consistent with this proposal. Therefore, at least in Drosophila, changes in the recombination rate in different lineages might cause deviations of the molecular-clock hypothesis and contribute to the overdispersion of the rate of synonymous substitution. In contrast, the change in the recombinational environment of the y gene has no detectable effect on the rate of amino acid replacement in the Yellow protein.     

Adaptive strategies of overwintering adults: Reproductive diapause and mating behavior in a grasshopper,Stenocatantops splendens (Orthoptera: Catantopidae)

Abstract To understand the adaptive strategies of the overwintering adults of Stenocatantops splendens, the mechanism of maintenance and termination of the reproductive diapause, the variation in mortality between overwintering females and males, and the mating strategy of the males were investigated. The results indicated that the adult reproductive diapause in natural conditions was mainly regulated by photoperiod in the fall – long photoperiods promoted reproductive development and short photoperiods maintained reproductive diapause, and the sensitivity of the overwintering adults to photoperiod was over before the end of the winter. When transferred from natural conditions to controlled laboratory conditions on dates from September through February, pre‐oviposition became increasingly shorter with increasingly deferred transfer dates regardless of photoperiod conditions. The adults treated with low temperature for 30 days in September through November had significantly shorter pre‐oviposition, suggesting that low temperatures in winter had an important role in the termination of reproductive diapause. The female had a significantly lower supercooling point than the male, which was related to their lower mortality after winter. In addition, observations of wild populations of the species indicated that mating behavior prior to winter and the duration of pre‐mating period were not affected by photoperiod; mating and sperm transfer were mostly completed by November. Compared with females only mating before winter, females mating in the spring had shorter life span, longer pre‐oviposition, lower hatching rate and laid fewer egg pods while showing no significant difference with regard to ovipositional interval, per pod number of eggs and nymph dry weight.     

Reevaluation of phylogeny in the Drosophila obscura species group based on combined analysis of nucleotide sequences.

The Drosophila obscura species group has served as an important model system in many evolutionary and population genetic studies. Despite the amount of study this group has received, some phylogenetic relationships remain unclear. While individual analysis of different nuclear, mitochondrial, allozyme, restriction fragment, and morphological data partitions are able to discern relationships among closely related species, they are unable to resolve relationships among the five obscura species subgroups. A combined analysis of several nucleotide data sets is able to provide resolution and support for some nodes not seen or well supported in analyses of individual loci. A phylogeny of the obscura species group based on combined analysis of nucleotide sequences from six mitochondrial and five nuclear loci is presented here. The results of several different combined analyses indicate that the Old World obscura and subobscura subgroups form a monophyletic clade, although they are unable to resolve the relationships among the major lineages within the obscura species group.     

Diapause and overwintering of two spruce bark beetle species

Diapause, a strategy to endure unfavourable conditions (e.g. cold winters) is commonly found in ectothermic organisms and is characterized by an arrest of development and reproduction, a reduction of metabolic rate, and an increased resistance to adversity. Diapause, in addition to adaptations for surviving low winter temperatures, significantly influences phenology, voltinism and ultimately population growth. We review the literature on diapause and overwintering behaviour of two bark beetle species that affect spruce‐dominated forests in the northern hemisphere, and describe and compare how these strategies can influence population dynamics. The European spruce bark beetle Ips typographus (L.) (Coleoptera, Curculionidae) is the most important forest pest of Norway spruce in Europe. It enters an adult reproductive diapause that might be either facultative or obligate. Obligate diapausing beetles are considered strictly univoltine, entering this dormancy type regardless of environmental cues. Facultative diapausing individuals enter diapause induced by photoperiod, modified by temperature, thus being potentially multivoltine. The spruce beetle Dendroctonus rufipennis (Kirby) (Coleoptera: Curculionidae) infests all spruce species in its natural range in North America. A facultative prepupal diapause is averted by relatively warm temperatures, resulting in a univoltine life cycle, whereas cool temperatures induce prepupal diapause leading to a semivoltine cycle. An adult obligate diapause in D. rufipennis could limit bi‐ or multivoltinism. We discuss and compare the influence of diapause and overwinter survival on voltinism and population dynamics of these two species in a changing climate and provide an outlook on future research.     

REPRODUCTIVE DIAPAUSE IN TYPHLODROMUS OCCIDENTALIS NESBITT (ACARINA: PHYTOSEIIDAE) FROM SOUTHERN NEW SOUTH WALES

Abstract
Studies on the reproductive diapause of an introduced insecticide-resistant strain of the phytoseiid predator, Typhlodromus occidentalis Nesbitt, from southern New South Wales are reported. Diapause was induced in 43–59% of predators when reared at 16 or 19°C under photophases of 10–12 h. No diapause induction response curve with changing photophase was evident. Diapause in overwintering predators was readily terminated by exposure to warm temperatures and a long photophase. The rapidity of diapause termination increased progressively during winter. The apparent loss of dependency upon reproductive diapause as an overwintering strategy in this strain of T. occidentalis is discussed.     

Dispersion of flightless adults of the Asian lady beetle, Harmonia axyridis, in greenhouses containing cucumbers infested with the aphid Aphis gossypii: effect of the presence of conspecific larvae

Nezara viridula (L.) (Heteroptera: Pentatomidae) recently expanded its distribution range in Japan and reached Osaka. In the southern temperate zone, the species overwinters in the adult stage and reproductive diapause is associated with a body colour change from green to russet. In Osaka, the reproductive diapause is only induced in September–October and nymphs from late egg masses are destined to die during winter. However, the fate of adults emerging late in the season remained unknown. Survival, body colour change, and post‐diapause reproduction were studied under quasi‐natural conditions in Osaka in 1999–2000 in those adults that attained adulthood as late as in November. Two experimental cohorts were used: in the Outdoor cohort, insects were reared outdoors starting in their second instar (28 September); in the Laboratory cohort, nymphs and subsequently adults were reared from the same day under diapause‐inducing conditions (L10:D14 at 25 °C), then acclimatized (5 days at 20 °C and 5 days at 15 °C; L10:D14) and transferred outdoors on 1 December. Adults in both cohorts did not reproduce in autumn and survived the winter with a low mortality. More than 20% of adults in the Outdoor cohort failed to change body colour from green to russet during winter, apparently because of the low ambient temperature, suggesting that the environmental conditions required for colour change do not completely coincide with those required for diapause induction, and that the colour of the adults is not always a reliable indicator of diapause in this species. After overwintering, females from the Outdoor cohort produced significantly fewer egg masses and eggs and had a significantly shorter period of oviposition than females that entered diapause under short‐day conditions in the Laboratory cohort. Thus, if progeny from the late egg masses attain adulthood late in the autumn, these adults have high chances of successful overwintering, but their reproductive output after the winter diapause is significantly reduced.     

The utilization of naturally occurring yeasts by Drosophila species, using chemically defined substrates

The ability of three Drosophila species; Drosophila obscura, D. subobscura and D. phalerata, to utilize a range of naturally occurring yeasts as food sources was determined. The yeasts used were the dominant species of two commonly used breeding sites, Sorbus aucuparia fruits and the fruiting bodies of the fungus Phallus impudicus. The range of yeast species used was found to reflect the degree ot substrate specialization exhibited by these three species, with D. subobscura being broader niched than the other two species. There was an indication that D. phalerata was adapted to those yeasts normally found on its natural breeding site, Phallus impudicus. Many factors were found to affect the nutritional adequacy of the yeasts, including their growth period, the larval density, the presence of competitors and particularly the chemical composition of the basal growth medium.