Sex chromosome variation and cytotaxonomy of the onchocerciasis vector Simulium squamosum in Cameroon and Nigeria

On the basis of sex chromosome variation, three cytotypes of Simulium squamosum (Enderlein) (Diptera: Simuliidae) are described from Cameroon and Nigeria. Simulium squamosum A is the typical form as originally described by Vajime & Dunbar (1975) with chromosome I as the sex chromosome. It occurs throughout most of Cameroon and south-east Nigeria. A second cytotype, S. squamosum B, is described from the river Sanaga (Cameroon). It also has chromosome I as the sex chromosome, but the nature of the sex differential region is different. Simulium squamosum C has no sex-linked chromosomal rearrangements. It is widespread in Nigeria and occurs near Mount Cameroon, where it seems to hybridize with S. squamosum A.     

Turnover of Sex Chromosomes in the Stickleback Fishes (Gasterosteidae)

Diverse sex-chromosome systems are found in vertebrates, particularly in teleost fishes, where different systems can be found in closely related species. Several mechanisms have been proposed for the rapid turnover of sex chromosomes, including the transposition of an existing sex-determination gene, the appearance of a new sex-determination gene on an autosome, and fusions between sex chromosomes and autosomes. To better understand these evolutionary transitions, a detailed comparison of sex chromosomes between closely related species is essential. Here, we used genetic mapping and molecular cytogenetics to characterize the sex-chromosome systems of multiple stickleback species (Gasterosteidae). Previously, we demonstrated that male threespine stickleback fish (Gasterosteus aculeatus) have a heteromorphic XY pair corresponding to linkage group (LG) 19. In this study, we found that the ninespine stickleback (Pungitius pungitius) has a heteromorphic XY pair corresponding to LG12. In black-spotted stickleback (G. wheatlandi) males, one copy of LG12 has fused to the LG19-derived Y chromosome, giving rise to an X₁X₂Y sex-determination system. In contrast, neither LG12 nor LG19 is linked to sex in two other species: the brook stickleback (Culaea inconstans) and the fourspine stickleback (Apeltes quadracus). However, we confirmed the existence of a previously reported heteromorphic ZW sex-chromosome pair in the fourspine stickleback. The sex-chromosome diversity that we have uncovered in sticklebacks provides a rich comparative resource for understanding the mechanisms that underlie the rapid turnover of sex-chromosome systems.     

Development of Onchocerca volvulus microfilariae injected into Simulium species from Cameroon

Microfilariae (mff) of the savanna and forest strains of Onchocerca volvulus (Leuckart) were injected intrathoracically into adult females of Simulium damnosum Theobald sensu stricto, S.sirbanum Vajime & Dunbar, S.squamosum Enderlein and S.mengense Vajime & Dunbar. Nine days post infection (pi) 27-29% of the savanna mff and 31-38% of the forest strain had developed to third-stage larvae (L3), irrespective of the fly species, size or injection dose (5, 10 or 15 mff). Savanna flies supported the development of forest O.volvulus better than forest flies, in contrast to the results after per os infections. Therefore, in these four species of the S.damnosum complex from Cameroon, the peritrophic membrane is considered to be the main factor limiting the success rate of microfilarial development following the ingestion of blood infections, while the fly's haemolymph and intracellular environment play minor roles.     

Cytogenetical analysis of the Simulium damnosum complex (Diptera: Simuliidae) in Guinea Bissau

Abstract. In a 3-year study during 1990–92, larval collections of the Simulium damnosum complex from the River Corubal system in Guinea Bissau revealed that the only sibling species present were S. sirbanum and the Konkouré form of S. konkourense , but not S. damnosum s.s. which had been found at some of the localities (Saltinho and Cusselinta) in a previous survey by Quillévéré et at. (1981). Their differential distributions were seasonally consistent between years. S. sirbanum was concentrated in the upper reaches of the Corubal, moving downstream at the end of the dry season to exploit new breeding grounds, whereas S. konkourense seemed to be concentrated in the lower reaches and moved upstream in the middle of the dry season.
Since the previous survey, it appears that S. konkourense has largely replaced S. sirbanum and S. damnosum s.s. in the lower reaches of the Corubal. S. sirbanum was consistently associated with man-biting behaviour and, although infection studies were not performed, it is likely that S. sirbanum is the main vector of onchocerciasis in Guinea Bissau as S. konkourense does not seen to be anthropophilic.
Among five polymorphic inversions in S. sirbanum , the frequency of inversion IL-B increased during the dry season; IL-2 and IIL-7 showed homozygous disadvantage whereas IIL-3 and IIIL-6 did not; IS-2 tended to be fixed in all populations. Simulium konkourense populations in the River Corubal differ from those found upstream in its tributary the River Koumba, in the Fouta Djallon of Guinea, by their sex chromosomes (having no sex-linked inversions) and biting preferences, indicating that the populations are not freely interbreeding. Inversion IIIL-X was found for the first time in populations of S. konkourense outside the Fouta Djallon area and a new inversion IIL-Y was identified in samples from the Rivers Corubal and Féfiné.     

Chromosome studies in the red howler monkey, Alouatta seniculus stramineus (Platyrrhini, Primates): description of an X1X2Y1Y2/X1X1X2X2 sex-chromosome system and karyological comparisons with other subspecies

In the red howler monkey, Alouatta seniculus stramineus (2n = 47, 48, or 49), variations in diploid chromosome number are due to different numbers of microchromosomes. Males exhibit a Y;autosome translocation involving the short arm of an individual biarmed autosome. Consequently, the sex-chromosome constitution in the male is X1X2Y1Y2, with X1 representing the original X chromosome, X2 the biarmed autosome (No. 7), Y1 the Y;7p translocation product, and Y2 the acrocentric homolog of 7q. In the first meiotic division, a quadrivalent with a chain configuration can be observed in spermatocytes. Females have an X1X1X2X2 sex-chromosome constitution. Chromosome heteromorphisms were observed in pair 13, due to a pericentric inversion, and pair 19, due to the presence of constitutive heterochromatin. Microchromosomes, which varied in number between individuals, were also heterochromatic. NOR-staining was observed at two separate sites on a single chromosome pair (No. 10). A comparison of A.s. stramineus with A.s. macconnelli shows that these two subspecies have identical diploid chromosome numbers (47, 48, or 49), again due to a varying number of microchromosomes, and that they share a similar sex-chromosome constitution. Their karyotypes, however, are not identical, but can be derived from each other by a reciprocal translocation. Further comparisons with other A. seniculus subspecies reported in the literature indicate that this taxon is not karyologically uniform and that substantial chromosome shuffling has occurred between populations that have been considered to be subspecies by taxonomic criteria based on their morphometric attributes.     

Inversion polymorphism in populations of Drosophila lummei, Hackman]

Four population inversions have been revealed in the chromosomes set of Drosophila lummei, Hackman, two of them being found for the first time. The new inversions (5S and 5R) are only typical of individuals from the Finnish population. The lines analysed are heterogeneous for inversions 4R and 4S. Individuals with the inversion 4R outnumber those with the 4S. In the USSR we found no predominance of these inversions in the D. lummei populations. The breaking points of the new inversions (5S and 5R) were located, and the boundaries of earlier described inversions in chromosome 4 were specified.     

Chromosomal inversion polymorphism of Anopheles funestus from forest villages of South Cameroon.

The polymorphism of paracentric inversions of Anopheles funestus polytene chromosomes was studied in three villages (Nkoteng, Obala, and Simbock) located in a forest area of South Cameroon in order to analyse the genetic structure of these populations. A total of 146-210 chromatids could be scored from specimens collected over about two years. A low degree of chromosomal polymorphism was observed with two floating inversions on chromosomal arm 2 (2h and 2d), and three fixed arrangements on arms 3 (3a and 3b), and 5 (5a). Such arrangement of inversions has never been recorded elsewhere so far. The chromosome analysis indicated that the population from Obala was in Hardy-Weinberg equilibrium, whereas the samples from Nkoteng and Simbock showed a significant excess and deficit of heterokaryotypes, respectively. Significant differences in inversion frequencies on chromosomal arm 2 among villages lying in contrasting eco-climatic settings suggested an adaptive role of these inversions.     

Heterogeneity of constitutive heterochromatin in somatic Syrian hamster chromosomes.

Syrian hamster constitutive heterochromatin was analyzed for C-band distribution and for BrU late-replication pattern. Characteristic for this species is relatively large amounts of sex-chromosome and autosomal heterochromatin. The distribution of constitutive heterochromatin was determined. The long term of the X chromosome, the whole Y, the short arms of 8 autosomal pairs, the long arm of the smallest metacentric pair, and the centromeric regions of 12 pairs stained intensely dark on C-band preparations. In contrast to the heterochromatin in the centromeric regions, the autosomal short-arm heterochromatin has an increased susceptibility to the denaturation process, as indicated by prolonged exposure to NaOH or Ba(OH)2. Such further exposure to denaturing agents results in an intense dark stain only on the sex-chromosome heterochromatin and centromeric regions of the autosomes. The BrdU late-replication pattern demonstrated that the late-replicating regions correspond to C-bands. Centromeric regions replicate late in the S phase; however, no centromeric region is among the latest replicating segments of the complement. Centromeric and noncentromeric heterochromatin are two distinct categories of constitutive heterochromatin.     

Cytotaxonomy,morphology and molecular systematics of the Bioko form of Simulium yahense (Diptera: Simuliidae)

Cytotaxonomic analysis of the polytene chromosomes from larvae of the Simulium damnosum Theobald complex from the island of Bioko in Equatorial Guinea is reported, and a new endemic cytoform is described. Chromosomally this cytoform is close to both S. squamosum (Enderlein) and S. yahense Vajime & Dunbar, but is not identical to either. However, it is morphologically and enzymatically identical to S. yahense. The Bioko form was also found to differ from other cytoforms of the S. damnosum complex in West Africa in the copy number or RFLP pattern of several different repetitive DNA sequences. It is clear that the Bioko form is genetically distinct from other populations of the S. damnosum complex, and whilst it is closest to S. yahense, it shows features that suggest a high degree of geographical and genetic isolation. Such isolation is an important consideration in the assessment of the potential for onchocerciasis vector eradication on Bioko.     

The evolution of reproductive systems and sex-determining mechanisms within rumex (polygonaceae) inferred from nuclear and chloroplastidial sequence data

The genus Rumex includes hermaphroditic, polygamous, gynodioecious, monoecious, and dioecious species, with the dioecious species being represented by different sex-determining mechanisms and sex-chromosome systems. Therefore, this genus represents an exceptional case study to test several hypotheses concerning the evolution of both mating systems and the genetic control of sex determination in plants. Here, we compare nuclear intergenic transcribed spacers and chloroplast intergenic sequences of 31 species of Rumex. Our phylogenetic analysis supports a systematic classification of the genus, which differs from that currently accepted. In contrast to the current view, this new phylogeny suggests a common origin for all Eurasian and American dioecious species of Rumex, with gynodioecy as an intermediate state on the way to dioecy. Our results support the contention that sex determination based on the balance between the number of X chromosomes and the number of autosomes (X/A balance) has evolved secondarily from male-determining Y mechanisms and that multiple sex-chromosome systems, XX/XY1Y2, were derived twice from an XX/XY system. The resulting phylogeny is consistent with a classification of Rumex species according to their basic chromosome number, implying that the evolution of Rumex species might have followed a process of chromosomal reduction from x = 10 toward x = 7 through intermediate stages (x = 9 and x = 8).     

Assessment of rDNA IGS as a molecular marker in the Simulium damnosum complex

For five cytospecies of the Simulium damnosum Theobald complex of blackflies (Diptera: Simuliidae) from West Africa, both ends of the intergenic spacer region (IGS) of the rDNA have been sequenced with the aim of developing specific molecular markers. No specific differences in these two regions were detected between Simulium sanctipauli V. & D., Simulium sirbanum V. & D., Simulium soubrense V. & D., Simulium squamosum Enderlein and Simulium yahense V. & D., except in the number of A subrepeats at the 5' end of the IGS (two in S. squamosum and four or five in the others) and in position 310 of the 3' end (a C in S. squamosum and a G in the others). However, genetic distances within and between species overlapped. These DNA sequences had no strong phylogenetic signal, and the trees obtained were mostly unresolved. Although most sequences from S. squamosum clustered together, a few of them were more similar to those in other cytospecies. These results could be explained either by hybridization with genetic introgression or by ancestral polymorphism and recent speciation.     

Fluorescence in situ hybridization establishes homology between human and silvered leaf monkey chromosomes,reveals reciprocal translocations between chromosomes homologous to human Y/5, 1/9, and 6/16, and delineates an X1X2Y1Y2/X1X1X2X2 sex-chromosome system

We employed in situ hybridization of chromosome-specific DNA probes (“chromosome painting”) of all human chromosomes to establish homologies between the human and the silvered lead monkey karyotypes (Presbytis cristata 2n=44). The 24 human paints gave 30 signals on the haploid female chromosome set and 34 signals on the haploid male chromosome set. This difference is due to a reciprocal translocation between the Y and an autosome homologous to human chromosome 5. This Y/autosome reciprocal translocation which is unique among catarrhine primates has produced a X₁X₂Y₁Y₂/X₁X₁X₂X₂ sex-chromosome system. Although most human syntenic groups have been maintained in the silvered leaf monkey chromosomes homologous to human chromosomes 14 and 15, 21 and 22 have experienced Robertsonian fusions. Further, the multiple FISH signals provided by libraries to human chromosomes 1/9, 6/16 indicate that these chromosomes have been split by reciprocal translocations. G-banding analysis shows three different forms of chromosome 1 (X₂) which differ by a complex series of inversions in the 10 individuals karyotyped. Comparisons with the hybridization patterns in hylobatids (gibbons and siamang) demonstrate that resemblances in chromosomal morphology and banding previously taken to indicate a special phylogenetic relationship between gibbons and colobines are due to convergence. A. J. Phys. Anthropol. 102:315–327, 1997. © 1997 Wiley-Liss, Inc.     

Multiple sex-chromosome system in a loach fish

This study shows an X1X1X2X2/X1X2Y type of multiple sex-chromosome system in the genus Cobitis (Pisces, Ostariophysi). Observation of C-banded male meioses revealed that one arm of the neo-Y always associates end-to-end with one of the X chromosomes. This finding implies that a mechanism might be present that has prevented crossing-over between the ancestral X and Y chromosomes without morphological differentiation. This is the first reported case of multiple sex chromosomes among cyprinoid fish.     

Cytosystematics of the Simulium tuberosum group (Diptera: Simuliidae) in Thailand

The polytene chromosomes of 3347 larvae of the Simulium tuberosum group in Asia were analysed, representing the largest ever cytogenetic study of black flies in the Oriental Region. Band‐by‐band comparisons, relative to the established standard chromosome map for the subgenus Simulium, revealed 17 cytogenetically distinct taxa in Thailand, plus an 18th in China. Six of these taxa correspond to morphologically described species (S. doipuiense, S. rufibasis, S. setsukoae, S. tani, S. yuphae and S. weji). Recognition of the 18 taxa is based largely on unique inversions, either fixed or sex linked, primarily in the long arm of chromosome III. The greatest cytological diversity was discovered in the S. tani lineage, with ten cytoforms. This marked chromosomal diversification within S. tani is based largely on two inversions that have assumed different roles over evolutionary time, variously functioning in different combinations as fixed inversions, sex‐linked inversions and autosomal polymorphisms. Shared unique chromosomal features, relative to the subgeneric standard chromosome map, allowed evolutionary relationships among the cytotaxa to be inferred. Fluctuations in climate during the Pleistocene might have promoted differentiation of the Southeast Asian S. tuberosum group in isolated refugia such as mountains. © 2009 The Linnean Society of London, Zoological Journal of the Linnean Society, 2009, 155 , 289–315.     

A reciprocal translocation radically reshapes sex‐linked inheritance in the common frog

X and Y chromosomes can diverge when rearrangements block recombination between them. Here we present the first genomic view of a reciprocal translocation that causes two physically unconnected pairs of chromosomes to be coinherited as sex chromosomes. In a population of the common frog (Rana temporaria), both pairs of X and Y chromosomes show extensive sequence differentiation, but not degeneration of the Y chromosomes. A new method based on gene trees shows both chromosomes are sex‐linked. Furthermore, the gene trees from the two Y chromosomes have identical topologies, showing they have been coinherited since the reciprocal translocation occurred. Reciprocal translocations can thus reshape sex linkage on a much greater scale compared with inversions, the type of rearrangement that is much better known in sex chromosome evolution, and they can greatly amplify the power of sexually antagonistic selection to drive genomic rearrangement. Two more populations show evidence of other rearrangements, suggesting that this species has unprecedented structural polymorphism in its sex chromosomes.     

The mutational spectrum of procarbazine in Drosophila melanogaster.

The antineoplastic agent Procarbazine was tested for the induction of genetic damage in Drosophila melanogaster. The compound was administered to adult males by oral application. The following types of genetic damage were measured: (1) sex-linked recessive lethals; (2) dominant lethals; (3) total and partial sex-chromosome loss; and (4) translocations. Procarbazine is highly mutagenic in causing recessive lethal mutations in all stages of spermatogenesis. In sperm a clear-cut concentration-effect relationship is not apparent, but in spermatids such a relationship is obtained for mutation induction at low levels of procarbazine exposure, while at high concentrations the induction of recessive lethals is not a function of concentration. A low induction of total sex-chromosome loss (X,Y) and dominant lethals was observed in metabolically active germ cells (spermatids), but procarbazine failed to produce well-defined breakage events, such as partial sex-chromosome loss (YL,YS) and II-III translocations. The results obtained in Drosophila melanogaster are discussed and compared with the mutational pattern reported in the mouse after procarbazine treatment.     

The interaction of Schistosoma haematobium and S. guineensis in Cameroon

Interactions between schistosomes are complex with some different species being able to mate and hybridize. The epidemiology of schistosomiasis in specific areas of South West Cameroon has evolved remarkably over 30 years as a result of hybridization between Schistosoma guineensis and S. haematobium. Morphological and biological data suggest that S. haematobium replaced S. guineensis in areas of Cameroon through introgressive hybridization. Data are reported on the use of single stranded conformational polymorphism (SSCP) analysis of the nuclear ribosomal second internal transcribed spacer (ITS2) of individual schistosomes from hybrid zones of Cameroon. The data show that since 1990 S. haematobium has completely replaced S. guineensis in Loum, with S. haematobium and the recombinants still present in 2000. This study illustrates the complexities of the dynamics between S. haematobium and S. guineensis in South West Cameroon.     

Cytogenetic features of the blood-sucking blackfly Wilhelmia paraequina Puri (Diptera: Simuliidae) from Armenia

Three blackfly Wilhelmia paraequina populations of Armenia (rivers Debet and Megriget, channel Megri) were studied. 2n = 6: IS + IIL, IIS + IL, IIIS + IIIL. Cytological maps of polytene chromosomes were constructed. High inversion polymorphism (95.63%) was observed, with 2.6 inversions per individual. Three inversions proved to be associated with male development: Y1 was characterized by a combination of two heterozygous inversions, IIL-3 + IIL-1, while Y2 had another combination of heterozygous inversions, IIL-5 + IIL-3. The X chromosome had a standard homozygous IIL disk sequence. The populations were shown to be similar in autosomal polymorphism. A tendency for differentiation was observed with respect to frequencies and types of sex-linked inversions: the sex determination system was Y1X-XX in the Debet and Megriget populations and Y2X-XX in the channel (Megri) population. On the strength of these findings, one W. paraequina morphotype was assumed to involve two cytotypes, A (Debet and Megriget populations) and B (Megri population).