Sex chromosome polymorphism in a species of Belostoma (Belostomatidae, Heteroptera)

A population of Belostoma sp. polymorphic for the sex chromosome determining system has been analyzed. The fundamental karyotype of the species is 2n = 16 = 14 + XY (male), and at different times individuals 2n = 17 have been encountered in a low frequency (27% and 16%, respectively). Sex chromosome area measurements confirm that the original X chromosome of the XY system has fragmented into two unequal-sized chromosomes (X1 and X2). At male metaphase II, the sex univalents associate in a pseudotrivalent that can show different arrangements (in a chain, in a double-plate, or in other transitional arrangements). Their frequency varies among individuals. The present polymorphic population represents a direct evidence of a multiple sex chromosome system originating through fragmentation of the single X. The different kinds of arrangement of the three sex chromosomes at male metaphase II, and their frequency within each individual suggest that some forces are acting to achieve a double plate arrangement and a regular meiotic behaviour. The maintenance of the polymorphism during more than three years seems to indicate that the new chromosomal variant is neutral, or even could be selectively advantageous.Fil:Papeschi, A.G. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Karyotype evolution in Aeshna (Aeshnidae, Odonata)

The haploid DNA content of Aeshna confusa (2n = 27, n = 13 + XO, male). A. bonariensis (2n = 26, n = 12 + neo-XY, male) and A. cornigera planaltica (2n = 16, n = 7 + neo-XY, male) has been determined (2.16 ± 0.16 pg, 1.81 ± 0.17 pg, and 2.08 ± 0.08 pg, respectively). Despite the differences in chromosome size and number, differences in DNA content between species are not significant. The karyotypic analysis of Aeshna species leads to the conclusion that fusions between autosomes or autosome and the sex chromosome, are the only chromosome rearrangement that occurred during evolution. In the species here studied, fusions have taken place with a minimal loss of DNA; however, other species of the genus show important differences in genome size, which cannot only be justified by fusion events.Fil:Mola, L.M. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Papeschi, A.G. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Meiosis in Haematopinus Suis and Menacanthus Stramineus (Phthiraptera, Insecta)

Haemalopinus suis (Anoplura) and Menacanthus stramineus (Mallophaga) have been cytogenetically analyzed. Both species have 2n = 10, holokinetic chromosomes, and achiasmatic male meiosis. Bivalents orientate with their long axis perpendicular to the spindle fibers at melaphase I, and first anaphase is reductional. As in other species of Phthiraptera, male gametogenesis follows a particular pattern: each cell entering meiosis results in a cyst of 64 (in H. suis) and 32 (in M. stramineus) spermatozoa and 64/32 non‐functional cells (=pycnotic nuclei). The results are compared with those previously reported for Phthiraptera, and a new terminology for the different stages of male gametogenesis is proposed. The low chromosome number together with the achiasmatic nature of male meiosis and the mitotic divisions that follow meiosis may restrict the potential for genetic variability. This might be related to the high host specificity of these parasites. Copyright © 1993, Wiley Blackwell. All rights reservedFil:Papeschi, A.G. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Sex chromosome polymorphism in a species of Belostoma (Belostomatidae, Heteroptera)

A population of Belostoma sp. polymorphic for the sex chromosome determining system has been analyzed. The fundamental karyotype of the species is 2n = 16 = 14 + XY (male), and at different times individuals 2n = 17 have been encountered in a low frequency (27% and 16%, respectively). Sex chromosome area measurements confirm that the original X chromosome of the XY system has fragmented into two unequal-sized chromosomes (X1 and X2). At male metaphase II, the sex univalents associate in a pseudotrivalent that can show different arrangements (in a chain, in a double-plate, or in other transitional arrangements). Their frequency varies among individuals. The present polymorphic population represents a direct evidence of a multiple sex chromosome system originating through fragmentation of the single X. The different kinds of arrangement of the three sex chromosomes at male metaphase II, and their frequency within each individual suggest that some forces are acting to achieve a double plate arrangement and a regular meiotic behaviour. The maintenance of the polymorphism during more than three years seems to indicate that the new chromosomal variant is neutral, or even could be selectively advantageous.Fil:Papeschi, A.G. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Karyotype evolution in Aeshna (Aeshnidae, Odonata)

The haploid DNA content of Aeshna confusa (2n = 27, n = 13 + XO, male). A. bonariensis (2n = 26, n = 12 + neo-XY, male) and A. cornigera planaltica (2n = 16, n = 7 + neo-XY, male) has been determined (2.16 ± 0.16 pg, 1.81 ± 0.17 pg, and 2.08 ± 0.08 pg, respectively). Despite the differences in chromosome size and number, differences in DNA content between species are not significant. The karyotypic analysis of Aeshna species leads to the conclusion that fusions between autosomes or autosome and the sex chromosome, are the only chromosome rearrangement that occurred during evolution. In the species here studied, fusions have taken place with a minimal loss of DNA; however, other species of the genus show important differences in genome size, which cannot only be justified by fusion events.Fil:Mola, L.M. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Papeschi, A.G. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Meiosis in Haematopinus Suis and Menacanthus Stramineus (Phthiraptera, Insecta)

Haemalopinus suis (Anoplura) and Menacanthus stramineus (Mallophaga) have been cytogenetically analyzed. Both species have 2n = 10, holokinetic chromosomes, and achiasmatic male meiosis. Bivalents orientate with their long axis perpendicular to the spindle fibers at melaphase I, and first anaphase is reductional. As in other species of Phthiraptera, male gametogenesis follows a particular pattern: each cell entering meiosis results in a cyst of 64 (in H. suis) and 32 (in M. stramineus) spermatozoa and 64/32 non‐functional cells (=pycnotic nuclei). The results are compared with those previously reported for Phthiraptera, and a new terminology for the different stages of male gametogenesis is proposed. The low chromosome number together with the achiasmatic nature of male meiosis and the mitotic divisions that follow meiosis may restrict the potential for genetic variability. This might be related to the high host specificity of these parasites. Copyright © 1993, Wiley Blackwell. All rights reservedFil:Papeschi, A.G. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Mitotic and meiotic chromosomes of Artemia (Branchiopoda) from populations of La Pampa Province, Argentina

Three populations of Artemia from La Pampa Province, Argentina, were cytogenetically analyzed: Salinas Grandes de Hidalgo, Laguna Callaqueo, and Laguna Colorada Chica. Both mitotic and male meiotic chromosomes were studied. All 3 populations share the same diploid (2n = 44) and/or haploid chromosome number (n = 22), and hence were determined as Anemia persimilis Piccinelli and Prosdocimi. Although bivalents decrease gradually in size, differences among the larger and the smaller ones were apparent. So far, no cytogenetic differences have been detected among the 3 populations. Chromosomes with metacentric, submetacentric, and telocentric morphology were detected in mitotic prometaphase cells, suggesting that chromosomes of Anemia may be monocentric.Fil:Rodríguez Gil, S. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Papeschi, A.G. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Cohen, R.G. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Possible origin of polymorphism for chromosome number in the assassin bug Zelurus femoralis longispinis (Reduviidae: Reduviinae)

In this study, we analysed a population of Zelurus femoralis longispinis polymorphic for chromosomal number. The fundamental karyotype of this subspecies is 2n=22=20A+XY (male), but individuals with 2n=23=20A+XY+extra chromosome have been found at high frequency and collected at different time periods. We examined male meiotic behaviour, average length as percentage of the sex chromosomes, the content, distribution and composition of heterochromatin, and the number and location of ribosomal DNA in the two cytotypes found. The meiotic behaviour of the extra chromosome was highly regular and similar to that of sex chromosomes. The average length of the sex chromosomes in individuals not carrying the extra chromosome was significantly greater than in those carrying it. The results support a hypothesis that the extra chromosome might have originated by fragmentation of the original X chromosome into two unequal-sized chromosomes (X1 and X2), leading to an X1X2Y multiple system. Maintenance of the polymorphism with time appears to indicate that the new chromosomal variant is neutral or at least not detrimental, or that it could be selectively advantageous. This polymorphic population represents direct evidence of a multiple sex chromosome system originating through fragmentation of a single X in Reduviidae as well as in Heteroptera. © 2013 The Linnean Society of London.Fil:Poggio, M.G. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Provecho, Y.M. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Papeschi, A.G. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Bressa, M.J. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina