Faithful chromosome segregation in all eukaryotes relies on centromeres, the chromosomal sites that recruit kinetochore proteins and mediate spindle attachment during cell division. The centromeric histone H3 variant, CenH3, is the defining chromatin component of centromeres in most eukaryotes, including animals, fungi, plants, and protists. In this study, using detailed genomic and transcriptome analyses, we show that CenH3 was lost independently in at least four lineages of insects. Each of these lineages represents an independent transition from monocentricity (centromeric determinants localized to a single chromosomal region) to holocentricity (centromeric determinants extended over the entire chromosomal length) as ancient as 300 million years ago. Holocentric insects therefore contain a CenH3-independent centromere, different from almost all the other eukaryotes. We propose that ancient transitions to holocentricity in insects obviated the need to maintain CenH3, which is otherwise essential in most eukaryotes, including other holocentrics.

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Recurrent loss of CenH3 is associated with independent transitions to holocentricity in insects

The evolution of heteromorphic sex chromosomes creates a genetic condition favoring the invasion of sex-ratio meiotic drive elements, resulting in the biased transmission of one sex chromosome over the other, in violation of Mendel's first law. The molecular mechanisms of sex-ratio meiotic drive may therefore help us to understand the evolutionary forces shaping the meiotic behavior of the sex chromosomes. Here we characterize a sex-ratio distorter on the X chromosome (Dox) in Drosophila simulans by genetic and molecular means. Intriguingly, Dox has very limited coding capacity. It evolved from another X-linked gene, which also evolved de nova. Through retrotransposition, Dox also gave rise to an autosomal suppressor, not much yang (Nmy). An RNA interference mechanism seems to be involved in the suppression of the Dox distorter by the Nmy suppressor. Double mutant males of the genotype dox; nmy are normal for both sex-ratio and spermatogenesis. We postulate that recurrent bouts of sex-ratio meiotic drive and its subsequent suppression might underlie several common features observed in the heterogametic sex, including meiotic sex chromosome inactivation and achiasmy.

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A sex-ratio Meiotic Drive System in Drosophila simulans. II: An X-linked Distorter

One of the stronger empirical generalizations to emerge from the study of genetic systems is that achiasmate meiosis, which has evolved 25–30 times, is always restricted to the heterogametic sex in dioecious species, usually the male. Here we collate data on quantitative sex differences in chiasma frequency from 54 species (4 hermaphroditic flatworms, 18 dioecious insects and vertebrates and 32 hermaphroditic plants) to test whether similar trends hold. Though significant sex differences have been observed within many species, only the Liliaceae show a significant sexual dimorphism in chiasma frequency across species, with more crossing over in embryo mother cells than in pollen mother cells; chiasma frequencies are unrelated to sex and gamety in all other higher taxa studied. Further, the magnitude of sexual dimorphism, independent of sign, does not differ among the three main ecological groups (dioecious animals, plants, and hermaphroditic animals), contrary to what would be expected if it reflected sex-specific selection on recombination. These results indicate that the strong trends for achiasmate meiosis do not apply to quantitative sex differences in recombination, and contradict theories of sex-specific costs and benefits. An alternative hypothesis suggests that sex differences may be more-or-less neutral, selection determining only the mean rate of recombination. While male and female chiasma frequencies are more similar than would be expected under complete neutrality, a less absolute form of the hypothesis is more difficult to falsify. In female mice the sex bivalent has more chiasmata for its length than the autosomes, perhaps compensating for the absence of recombination in males. Finally, we observe that chiasma frequencies in males and females are positively correlated across species, validating the use of only one sex in comparative studies of recombination.

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Sex differences in recombination

Like mammals, Lepidoptera possess female-specific sex chromatin. In a compilation of new and published data, 81 % of the 238 investigated Lepidoptera species display one or more heterochromatin bodies infemale somatic interphase cells, but not in male cells. In contrast with the similar phenomenon in mammals, this sex-specific heterochromatin does notfunction as a dosage compensation mechanism. Most Lepidoptera have a WZ/ZZ sex chromosome mechanism, and the sex chromatin is derived from the univalent W sex chromosome. Sex chromatin is regarded as an indicator of an advanced stage of Wchromosome evolution. In species with a Z/ZZ sex chromosome mechanism, loss of the Wchromosome is accompanied by loss ofthefemale-specific heterochromatin. Since sex chromatin can be discerned easily in interphase nuclei, and especially so in the highly polyploid somatic cells, it is a useful marker for diagnosing chromosomal sex of embryos and larvae, and of identifying sex chromosome aberrations in mutagenesis screens. All species with sex chromatin belong to theDitrysia, the main clade of Lepidoptera that contains more than 98 % of all extant species. Sex chromatin has not been reportedfor clades that branched off earlier. The nonditrysian clades share this character with Trichoptera, a sister group of the Lepidoptera. We propose that Lepidoptera originally had a Z/ZZ sex chromosome mechanism like Trichoptera; the WZ/ZZ sex chromosome mechanism evolved later in the ditrysian branch of Lepidoptera. Secondary losses of the W chromosome account for the sporadically occurring Z/ZZ sex chromosome systems in ditrysianfamilies. The lepidopteran sex chromatin, therefore, appears to mirror thefull evolutionary life cycle of a univalent sex chromosomefrom its birth through heterochromatinization to sporadic loss.

Sex Chromatin in Lepidoptera

Distribution of Odonata sampling effort in Brasil – basis for choosing prioritary areas for faunistic inventories. There is an urgent need to prioritize conservation areas, so that the invested resources protect as much biodiversity as possible, especially in developing countries. A way to select such areas is to estimate the biodiversity distribution but this strategy is constrained by the lack of knowledge and/or by the inaccessibility of the available information on species distribution. Based on a database on Odonata distribution in Brazil, it is shown that the sampling-effort and the largest numbers of recorded odonate species and genera are concentrated in areas with large number of researchers. The 6203 records employed here are distributed in only 29% of the national territory. The species richness is concentrated mainly in the states of Rio de Janeiro, Minas Gerais and Sao Paulo, in the Amazon River varzea and in some isolated points, such as Cuiaba (Mato Grosso state). We suggest that the sampling and inventory efforts are increased for Odonata, especially in the Brazilian northeast and areas in the Brazilian and Guyana Shields, for which there is practically no information available.

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Distribuição do esforço de coleta de Odonata no Brasil - subsídios para escolha de áreas prioritárias para levantamentos faunísticos

The morphological and kinetical features of the odonate sex chromosomes are reviewed and trends of the evolution of different modes of sex determination in the order are considered. The cytological conditions in the familyGomphidae are of particular importance for the understanding of the evolution of sex determination in dragonflies and are discussed in detail.

Sex chromosomes and sex determining mechanisms in Odonata, with a review of the cytological conditions in the family Gomphidae, and references to the karyotypic evolution in the order.

The views of earlier authors on the karyotypic evolution in dragonflies are criticised and a new hypothesis is proposed. It is based on integration of independent karyological, morphological and paleontological evidence, is accounting for the origin of any chromosome number, and is in agreement with the evidence found and the opinions expressed so far on the course of the karyotype numerical evolution in the other insect orders possessing holokinetic chromosomes. It seems most likely that the present numerical variation in odonate karyotypes has developed through the occurrence of breaks (leading to haploid numbers 10 to 15) and fusions (leading to the complements 3 to 7) in ancestral forms which had n=9. Low-n complements have been reported for tropical species only, whereas secondarily reduced high-n complements so far have been recorded only in dragonflies from the Temperate Region. The biological significance of the karyotype variation within a species is considered and a list of the known dragonfly hybrids is given.

Considerations on the evolution of the chromosome complement in Odonata.

The fragmentation of autosomes in the karyotypes ofEnallagma cyathigerum (Charp.) (n=14→15),Mecistogaster sp. (n=15),Hetaerina rosea Sel. (n=14),Libellula depressa L. (n=12→13),Orthetrum coerulescens (Fabr.) (n=12→13),Diplacodes bipunctata (Br.) (n=13→15) andD. haematodes (Burm.) (n=12→13) is discussed. Original material ofMecistogaster andHetaerina was not available. Fragmentations were found to be the only way in which the recombination index is obligatorily changed (increased) in dragonflies. In at least five out of the seven cases considered the chromosome number is not stabilised: cells in which fragmentation occurred and those in which it did not are found in the same individual (Enallagma, Libellula, Orthetrum, Diplacodes). Fragmentation results in an increase of chromosome number (a) up to the family type number level (Libellula, Orthetrum, D. haematodes), (b) above the latter (Hetaerina) and (c) above any chromosome number ever reported in dragonflies (Mecistogaster, D. bipunctata). In our material the element or elements formed by fragmentation have always the size and usually also the heterochromatic features of the m-chromosomes, irrespective of the presence or absence of the latter in the original complement. A review of species in which fusions of autosomes were recorded or can be assumed is given.Orthetrum brachiale (Beauv.) (n=11) is added to the list. The situation in this species and inSympetrum eroticum Sel. is discussed in detail. The most essential differences between the fusion of (an) autosome (s) with the sex element on one hand (cf.Kiauta, 1969), and the fusion of two or more autosomes on the other, lies in the observations: (a) that in the former case, fused and unfused complements occur in one individual, whereas the latter is specifically characteristic and occurs in all cells, all individuals and all populations of the species, and (b) that autosomal fusion results in an increase of chiasma frequency, due to which the recombination index in secondarily low-n complements remains the same as it was in the primary high-n sets, or becomes even higher (O. brachiale, S. eroticum). Autosomal fragmentation (found so far in some advanced forms only) is considered as a character of phylogenetic advancement. Autosomal fusion, on the other hand, does not have any relation with phylogeny.

Autosomal fragmentations and fusions in odonata and their evolutionary implications

Male and female karyotypes are described of material from eastern Switzerland (2n d -60, 2n ? = 59; d XX/ 9 XO sex determination). At d meiotic metaphase ehiasma frequency is unusually high, more than half of the bivalents having two chiasmata; the recombination index is thus much higher than in other n = 30 limnephilide species. The significance of this situation is discussed in the light of the distribution, infraspeciation and autecology of this species. Details on habitat and habits and on larval case structure and morphology are presented, and it is suggested that the increased recombination index compensates for the reduction of gerietic flexibility caused by the sedentary larval habits.

Ecology, case structure, larval morphology and chromosomes of the caddis-fly, allogamus auricollis (Pictet, 1834), with a discussion on the variation of recombination indices in the stenophylacini (Trichoptera, Integripalpia: Limnephilidae)

Notes on karyotypic morphology are given for ten South American species of the dragonfly family Libellulidae that were either cytologically little known or not studied previously, viz. Dasythemis venosa (Burm.) (Brasil, n=13), Orthemis ferruginea (Fab.) (Peru, n=12), Micrathyria laevigata Calv. (Brasil, n=13), Erythrodiplax cleopatra Ris (Peru, n=13), E. connata connata (Burm.) (Chile, n=12). E. media Borror (Brasil, n=11), E. melanorubra Borror (Venezuela, n=13), Dythemis velox Hag. (Peru, n=13), Macrothemis declivata Calv. (Brasil, n=12) and M. imitans imitans Karsch (Brasil, n=13). A pair (bivalent) of m-chromosomes is present in all species.

Cytology of ten South American Libellulidae, with cytophylogenetic considerations on the genera Orthemis Hagen and Erythrodiplax Brauer (Odonata, Anisoptera)

B. Kiauta

Papers

Sex chromosomes and sex determining mechanisms in Odonata, with a review of the cytological conditions in the family Gomphidae, and references to the karyotypic evolution in the order.

Considerations on the evolution of the chromosome complement in Odonata.

Autosomal fragmentations and fusions in odonata and their evolutionary implications

Ecology, case structure, larval morphology and chromosomes of the caddis-fly, allogamus auricollis (Pictet, 1834), with a discussion on the variation of recombination indices in the stenophylacini (Trichoptera, Integripalpia: Limnephilidae)

Cytology of ten South American Libellulidae, with cytophylogenetic considerations on the genera Orthemis Hagen and Erythrodiplax Brauer (Odonata, Anisoptera)