TTAGG Telomeric Repeats in Chromosomes of Some Insects and Other Arthropods

We studied the occurrence of the TTAGG telomere repeats by fluorescence in situ hybridization (FISH) and Southern hybridization in ten insect species and two other arthropods. (TTAGG)n-containing telomeres were found in three Lepidoptera species, the silkworm Bombyx mori (in which the telomeric sequence was recently discovered), the flour moth Ephestia kuehniella, and the wax moth Galleria mellonella, in one species of Hymenoptera, the honey bee Apis mellifera, in one species of Coleoptera, the bark beetle Ips typographus, in one species of Orthoptera, the locust Locusta migratoria, and in a crustacean, the amphipod Gammarus pulex. They were absent in another species of Coleoptera, the mealworm Tenebrio molitor, in two representatives of Diptera, Drosophila melanogaster and Megaselia scalaris, in a species of Heteroptera, the bug Pyrrhocoris apterus and in a spider, Tegenaria ferruginea. Our results, which confirm and extend earlier observations, suggest that (TTAGG)n was a phylogenetically ancestral telomere motif in the insect lineage but was lost independently in different groups, being replaced probably by other telomere motifs. In the Coleoptera this must have happened rather recently as even members of the same family, Curculionidae, differ with respect to the telomeric DNA

Neo sex chromosomes, colour polymorphism and male-killing in the African queen butterfly, Danaus chrysippus (L.)

Danaus chrysippus (L.), one of the world’s commonest butterflies, has an extensive range throughout the Old-World tropics. In Africa it is divided into four geographical subspecies which overlap and hybridise freely in the East African Rift: Here alone a male-killing (MK) endosymbiont, Spiroplasma ixodetis, has invaded, causing female-biased populations to predominate. In ssp. chrysippus, inside the Rift only, an autosome carrying a colour locus has fused with the W chromosome to create a neo-W chromosome. A total of 40–100% of Rift females are neo-W and carry Spiroplasma, thus transmitting a linked, matrilineal neo-W, MK complex. As neo-W females have no sons, half the mother’s genes are lost in each generation. Paradoxically, although neo-W females have no close male relatives and are thereby forced to outbreed, MK restricts gene flow between subspecies and may thus promote speciation. The neo-W chromosome originated in the Nairobi region around 2.2 k years ago and subsequently spread throughout the Rift contact zone in some 26 k generations, possibly assisted by not having any competing brothers. Our work on the neo-W chromosome, the spread of Spiroplasma and possible speciation is ongoing

Sex chromosome evolution in moths and butterflies

Lepidoptera, i.e. moths and butterflies, have a female heterogametic sex chromosome system, with most females having a WZ constitution while males are ZZ. Besides this predominant WZ/ZZ system, Z/ZZ, WZ1Z2/Z1Z1Z2Z2 and W1W2Z/ZZ systems also occur. Sex is determined by an unknown W-linked gene or genes in Bombyx mori, but by dosage-dependent and equally unknown Z-linked genes in all Z/ZZ species. The female-heterogametic sex chromosome system has been conserved for at least 180 milliion years in the phylogenetic branch that combines Lepidoptera and Trichoptera. The W chromosome, which is present in most lepidopteran species, was incorporated into the system much later, about 90-100 MYA. The Z chromosomes display conserved synteny, much like the Z in birds or the X in mammals. The W, on the other hand, is evolving rapidly. It is crammed with repetitive elements which appear to have a high turnover rate, but poor in or even devoid of protein-coding genes. It has frequently undergone fusion with autosomes or sporadically been lost altogether

Moth sex chromatin probed by comparative genomic hybridization (CGH)

Comparative genomic hybridization (CGH) with a probe mixture of differently labeled genomic DNA from females and males highlighted the W chromosomes in mitotic plates and the W chromatin in polyploid interphase nuclei of the silkworm Bombyx mori, the flour moth Ephestia kuehniella, and the wax moth Galleria mellonella. The overproportionate fluorescence signal indicated an accumulation of repetitive sequences in the respective W chromosomes. Measurements of the fluorescence signals revealed two components, one that is present also in male DNA (non-W chromosomes) and another one that is present only in or preponderantly in female DNA (W chromosomes). While the W chromosomes of E. kuehniella and G. mellonella had both components, that of B. mori appeared to lack the latter component. Our results show that CGH can be applied to obtain a first estimate of the sequence composition of sex chromosomes in species from which otherwise little is known on the molecular level.http://pubs.nrc-cnrc.gc.c

Molecular differentiation of sex chromosomes probed by comparative genomic hybridization

Comparative genomic hybridization (CGH) was used to identify and probe sex chromosomes in several XY and WZ systems. Chromosomes were hybridized simultaneously with FluorX-labelled DNA of females and Cy3-labelled DNA of males in the presence of an excess of Cot-1 DNA or unlabelled DNA of the homogametic sex. CGH visualized the molecular differentiation of the X and Y in the house mouse, Mus musculus, and in Drosophila melanogaster: while autosomes were stained equally by both probes, the X and Y chromosomes were stained preferentially by the female-derived or the male-derived probe, respectively. There was no differential staining of the X and Y chromosomes in the fly Megaselia scalaris, indicating an early stage of sex chromosome differentiation in this species. In the human and the house mouse, labelled DNA of males in the presence of unlabelled DNA of females was sufficient to highlight Y chromosomes in mitosis and interphase. In WZ sex chromosome systems, the silkworm Bombyx mori, the flour moth Ephestia kuehniella, and the wax moth Galleria mellonella, the W chromosomes were identified by CGH in mitosis and meiosis. They were conspicuously stained by both female- and male-derived probes, unlike the Z chromosomes, which were preferentially stained by the male-derived probe in E. kuehniella only but were otherwise inconspicuous. The ratio of female:male staining and the pattern of staining along the W chromosomes was species specific. CGH shows that W chromosomes in these species are molecularly well differentiated from the Z chromosomes. The conspicuous binding of the male-derived probe to the W chromosomes is presumably due to an accumulation of common interspersed repetitive sequences