Jon Martin

Copy number variation and transposable elements feature in recent, ongoing adaptation at the Cyp6g1 locus.

The increased transcription of the Cyp6g1 gene of Drosophila melanogaster, and consequent resistance to insecticides such as DDT, is a widely cited example of adaptation mediated by cis-regulatory change. A fragment of an Accord transposable element inserted upstream of the Cyp6g1 gene is causally associated with resistance and has spread to high frequencies in populations around the world since the 1940s. Here we report the existence of a natural allelic series at this locus of D. melanogaster, involving copy number variation of Cyp6g1, and two additional transposable element insertions (a P and an HMS-Beagle). We provide evidence that this genetic variation underpins phenotypic variation, as the more derived the allele, the greater the level of DDT resistance. Tracking the spatial and temporal patterns of allele frequency changes indicates that the multiple steps of the allelic series are adaptive. Further, a DDT association study shows that the most resistant allele, Cyp6g1-[BP], is greatly enriched in the top 5% of the phenotypic distribution and accounts for ∼16% of the underlying phenotypic variation in resistance to DDT. In contrast, copy number variation for another candidate resistance gene, Cyp12d1, is not associated with resistance. Thus the Cyp6g1 locus is a major contributor to DDT resistance in field populations, and evolution at this locus features multiple adaptive steps occurring in rapid succession.

Multiple sex determining regions in a group of related Chironomus species (Diptera:Chironomidae)

SummaryIn a group of Australian Chironomus species, sex determining regions involved in male heterogamety have been found at a minimum of five different chromosomal locations. In one case, Ch. oppositus A, sex determining regions have been identified on two different chromosomes. It is uncertain whether this is a polymorphism or indicates the existence of multiple sex determining regions. It is argued that all the published data on sites of male determining regions are compatible with the five locations found in this study. On the basis of the postulated phylogeny of the studied group, the location of the sex determining regions would appear to have changed during speciation in all cases except one. The significance of these changes in location during the speciation process is discussed.

Population variability in Chironomus (Camptochironomus) species (Diptera, Nematocera) with a Holarctic distribution: evidence of mitochondrial gene flow.

Phylogenetic analysis of DNA sequences from mitochondrial (mt) genes (Cytochrome b and Cytochrome oxidase I) and one nuclear gene (globin 2b) was used for the investigation of Nearctic and Palearctic populations representing four Chironomus species of the subgenus Camptochironomus, namely C. biwaprimus, C. pallidivittatus, C. tentans sensu stricto and C. dilutus (the last two species constitute Holarctic C. tentans sensu lato). Phenograms constructed on the basis of mt sequences were not congruent with trees based on nuclear genes, or with morphological and cytological data. The mt tree divided the populations by continental region, rather than by the species groupings recognized by the other data sets. The incongruence is explained by mt gene flow resulting from hybridization between the sympatric species on each continent. Calculation of divergence times, based on the sequence data, suggest that C. tentans (s.l.) and C. pallidivittatus have both been in North America for about 2.5 My.

A review of the genusChironomus (Diptera, Chironomidae)

Two cytologically distinguishable species have been found in material ofChironomus australis. These species have been calledCh. australis andCh. duplex. Both belong to thepseudothummi-cytological grouping because they have the chromosome arm combinations AE, BF, CD, G.Ch. duplex shows a modified arm pattern due to a tandem fusion of arm G to arm E.—The banding patterns of the polytene chromosomes of the two species are compared to each other and to the Australian standard species,Ch. oppositus. Ch. australis is very close cytologically toCh. oppositus, whileCh. duplex, which is considered a derived species because of the tandem fusion, shows a number of inversion differences from the morphologically similarCh. australis. Ch. duplex is polymorphic for six inversions, four of which are simple inversions, the other two are complex involving also the transposition of some bands.Ch. australis appears to be monomorphic.

A molecular assessment of the extent of variation and dispersal between Australian populations of the genus Archaeochlus Brundin (Diptera : Chironomidae)

Australian populations of the primitive podonomine genus Archaeochlus have a restricted distribution associated with seepage areas on granite outcrops in south-western and central Australia. These granite outcrops are disjunct and the intervening land offers no apparent suitable habitat for Archaeochlus. This paper uses the DNA sequence of the mitochondrial Cytb gene to assess the phylogenetic relationships between populations of three species, and to examine the hypothesis that these populations achieved their present distribution via ancient river systems that existed up until about 65 million years ago. The phylogeny supports the existence of the three species, but also raises the possibility that a population of A. brundini from Baladjie Rock may be specifically distinct. The phylogenetic relationships do not discriminate between the ancient river hypothesis and alternative explanations. However, estimation of the times of separation of the various populations within species indicates that these populations are too recent to have spread via the river systems. The results show that for each species the most northerly populations are the oldest and that there has been a southward spread of the populations. It is postulated that the southward spread may be a consequence of the increasing aridity of the northern parts of their range since the end of the tertiary.

Intercontinental karyotypic differentiation of Chironomus entis Shobanov, a holarctic member of the C. plumosus group (Diptera, Chironomidae).

Analysis of banding sequences of polytene chromosomes in Palearctic (Russian) and Nearctic (North American) Chironomus entis shows strong karyotype divergence between populations on the two continents. Four out of seven chromosomal arms in the North American C. entis karyotype are characterized by sequences found only in the Nearctic. In total, 44 banding sequences are now known for this species across the Holarctic, including 22 exclusively Palearctic, 6 Holarctic, and 16 exclusively Nearctic sequences. The degree of cytogenetic differentiation between Palearctic and Nearctic C. entis populations is an order of magnitude greater than differentiation among populations within either continent, but is only one third as great as the cytogenetic distance between the sibling species C. entis and C. plumosus. C. entis is the only sibling species of C. plumosus uncovered during cytological identification of Chironomus species from more than 50 North American lakes, indicating that the plumosus sibling-species group is much smaller in the Nearctic than in the Palearctic, where a dozen sibling species are known. Cytogenetic distance values calculated between Nearctic and Palearctic representatives of both C. entis and its sibling species C. plumosus are similar, but result from different patterns of karyotype divergence. New World C. entis is distinguished from Old World populations by the 16 uniquely Nearctic sequences, four of which occur in the homozygous state. In contrast, North American C. plumosus has fewer uniquely Nearctic sequences, and only one that occurs as a homozygote. However, four chromosomal arms in C. plumosus that are polymorphic in the Palearctic show fixation, or near fixation, of Holarctic sequences in the Nearctic C. plumosus karyotype. Thus, both the fixation of Holarctic sequences, and the occurrence or fixation of distinctly Nearctic sequences, contribute significantly to karyotype divergence. Patterns of karyotype divergence in Palearctic and Nearctic populations of different Holarctic chironomid species are discussed relative to intercontinental cytogenetic differentiation in other dipterans.

A phylogenetic study of sex determiner location in a group of Australasian Chironomus species (Diptera, Chironomidae)

Sex determination in a group of phylogenetically related Chironomus species, of the pseudothummi complex, from south-eastern Australia and New Zealand is male heterogametic, controlled by a male determiner. The male determiner has been located at least to the level of the chromosome arm in most members of this phylogenetic group. It varies in location among many of the species and there are some phylogenetic patterns discernable, which are discussed in relation to the possible origin of the sex determiner. There is a group of species, Ch. oppositus ff. oppositus and whitei, Ch. australis, Ch. alternans a and Ch. alternans c, which appear to be central to this phylogeny, in which the sex determiner is located near the centromere of the CD chromosome, the most common location in the Australasian group. This is different from the most common location, arm F, of the thummi complex in Europe and North America. There is also a group, comprising Ch. oppositus f. tyleri, Ch. cloacalis, Ch. alternans b and Ch. nepeanensis, in which the sex determiner is on arm G. The arm A sex determiners, found in Ch. tepperi, Ch. oppositus ff. whitei and connori, and Ch. occidentalis, may be of common origin or they may be independently derived, as must be the arm B (Ch. duplex) and arm F (Ch. oppositus f. whitei) sex determiners. In Ch. oppositus f. whitei, four different chromosomal locations for the sex determiner have been identified. It is not yet clear whether these represent an unstable polymorphism or indicate the existence of cryptic subgroupings within this form. Although the location of the sex determiners can be assigned to particular chromosome arms, the precise location cannot be determined, therefore the assumption of common origin may not always be correct. Also, this uncertainty means that it is impossible at present to differentiate between a complex system of sex determination and the possibility of a translocatable sex determiner as explanations of the variability in sex determiner location. The forms of Ch. oppositus are redefined and renamed to avoid confusion caused by the previous names.

Chromosomes and Continents

The high level of inversion polymorphism and, correspondingly, the abundance of inversion banding sequences (BSs) of polytene chromosomes in the banding sequence pool of Chironomus species permit scientists to reconstruct the cytogenetic evolution of the genus and to evaluate the role of structural rearrangements in the genome during population divergence and speciation. We performed a quantitative assessment of the important role of inversion polymorphism in the differentiation of natural populations and demonstrated the adaptive significance of different gene orders in populations of species occurring in different regions. For the first time, it has been shown that the BS pools of populations of the same species on different continents differed much in the sets and frequencies of gene inversion orders. BS pools of populations on each continent were found to contain continent-specific BSs in addition to sequences occurring on several continents. This intraspecies diversity of the linear organization of the genome is one of the major factors maintaining the evolutionary stability of species in dramatically different environments. In addition to endemic species-specific sequences, the BS pool of the genus Chironomus contains sequences common for different species, cytocomplexes, and continents. These sequences, termed basic sequences, are very important for reconstruction of genome divergence in the course of evolution. It is suggested that they are close to the initial primitive sequences existing on ancient supercontinents, whereas continent-specific BSs were formed after continent separation. Comparison of all currently known BSs in the sequence pool of the genus Chironomus showed that the genomes of the most distant species differed by more than 90 inversion breaks, causing changes of their linear structure. In such cases, conserved genome regions span about 10 bands.

A review of the genus Chironomus (Diptera, Chironomidae)

Analysis of the banding pattern of the salivary gland chromosomes of Chironomus tepperi indicates that, despite a somewhat modified male hypopygium, the relationships of this species are close to the other Australian species of the genus, particularly to Ch. oppositus. No inversion polymorphism has been found in Ch. tepperi and this, together with the relatively high chiasma frequency as measured at metaphase I, would appear to be an adaptation to provide genetic variability necessary for its colonizing ability.

The Cytology of Polypedilum Nubifer (Diptera: Chironomidae)

SUMMARYPolypedilum nubifer has four polytene chromosomes (2 n = 8). A standard chromosome map of the polytene chromosomes and the limits of 29 alternative sequences are described. Phylogenies for the sequences on arms C, E and G are proposed. Sequences C4, E3, Fl and G1 are apparently quite ancient, and therefore probably ancestral, because of their occurrence in widely separated populations in Australia and Israel. There were no common sequences in arms A1, B1 and D1 but sequences A1, B1 and D1 are the most widespread in Australia. P. nubifer is female heterogametic, females always being heterozygous for a heterodhromatinized differential segment 41cY-dY on arm G. On the basis of the differing frequencies of some of the arm G sequences, it is likely that the sex determining regions are included within the differential 41c-d ends.