Stability of a multiple X chromosome system and associated B chromosomes in birch aphids (Euceraphis spp.; Homoptera: Aphididae)

Autosomal dissociations are a common feature of aphid karyotype evolution, but multiple X chromosome systems are rare. Birch-feeding aphids of the genus Euceraphis, however, have X₁X₂O males as a general rule, X₁ being always much larger than X₂. Only one species has XO males, and this condition appears to be secondary. Most Euceraphis karyotypes also have one or more, usually heterochromatic, elements that occur in the same numbers in both males and females, yet behave like X chromosomes at male and female meiosis I. They appear to be supernumerary, non-functional X chromosomes, although showing greater within-species stability in size and number than typical B chromosomes. Euceraphis gillettei forms a separate group within the genus and feeds on alders (Alnus species), yet has a similar system, and the two most closely related genera, Symydobius and Clethrobius, also have additional chromosomal elements possibly representing non-functional X chromosomes. Thus the multiple X chromosome system in these aphids seems to be a primitive condition.     

Complex sex-determining mechanisms in three species of South American grasshoppers (Orthoptera,Acridoidea)

The karyotype of three species of South American grasshoppers are studied in this paper. Leiotettix sanguineus has two chromosome races, one of them with 2n=23 and an XO sex mechanism and the other, as far as we know limited to the Cerro Chato population, with 2n=22 and an XY sex mechanism. Leiotettix politus has two kinds of individuals, one with 2n=14 and XY sex chromosomes and the other 2n=13 and an X₁X₂Y mechanism. Dichroplus dubius presents 2n=21 and an X₁X₂Y sex chromosomes. One of the three specimens studied shows aberrant behaviour in the meiotic process.     

The founder effect theory: quantitative variation and mdg-1 mobile element polymorphism in experimental populations of Drosophila melanogaster

The chromosomes of 14 specimens of the genus Reithrodon from three different localities of Argentina and two localities of Uruguay were studied using G-and C-banding techniques. Specimens of Uruguay showed a karyotype of 2n=28 chromosomes having a large metacentric X, and a telocentric Y chromosome. This karyotype is very similar to that recently described in a sample from southern Brazil, differing only in the nature of the Y chromosome, which is metacentric in the Brazilian form. All specimens from Argentina showed a 2n=34 karyotype, differing from the Brazilian karyotype by two centric fusions, an acquisition of chromosome material, and at least one pericentric inversion, and by the telocentric nature of both the X and the Y chromosomes. G-and C-banding suggest that the metacentric gonosomes in the Brazilian form resulted from a double autosomal-X-Y Robertsonian translocation. The Uruguayan cytotype is interpreted as derived from a hypothetical neo-X/Y₁Y₂ ancestral form by the secondary loss of the Y₁ chromosome. The karyotypic differences between the Brazilian-Uruguayan and the Argentinian forms afford evidence of species differentiation. It is proposed to assign the former to Reithrodon typicus, and the later to R. auritus.     

Chromosome banding in amphibia

A cytogenetic study performed on a population of the South American leptodactylid frog Eleutherodactylus maussi revealed multiple sex chromosomes of the X₁X₁X₂X₂/X₁X₂Y (=XXAA/XXA^Y) type. The diploid chromosome number is 2n=36 in all females and 2n=35 in most males. The multiple sex chromosomes originated by a centric fusion between the original Y chromosome and a large autosome. In male meiosis the X₁X₂Y (=XXA^Y) multiple sex chromosomes form a classical trivalent configuration. E. maussi is the first species discovered in the class Amphibia that is distinguished by a system of multiple sex chromosomes. Only one single male was found in the population with 2n=36 chromosomes and lacking the Y-autosomal fusion. This karyotype (XYAA) is interpreted as the ancestral condition, preceding the occurrence of the Y-autosome fusion.by H.C. Macgregor     

Chromosomal polymorphisms involving telomere fusion,centromeric inactivation and centromere shift in the ant Myrmecia (pilosula) n=1

Detailed karyological surveys of the ant Myrmecia pilosula species group, which is characterized by the lowest chromosome number in higher organisms (2n=2), were attempted. We revealed that this species has developed highly complicated chromosomal polymorphisms. Their chromosome numbers are in the range 2n=2, 3, and 4, and six polymorphic chromosomes are involved, i.e., two for chromosome 1 (denoted as SM₁ and ST₁), three for chromosome 2 (A₂, A₂, and M₂), and M₍₁₊₂₎ for the 2n=2 karyotype. We suggested that these chromosomes were induced from a pseudo-acrocentric (A ₁ ^M ) and A₂ as follows: (1) A ₁ ^M SM₁ or ST₁ by two independent pericentric inversions; (2) A₂A₂M₂ by chromosomal gap insertion and centromere shift; and (3) ST₁+A₂M₍₁₊₂₎ by telomere fusion, where (3) means that the 2n=2 karyotype was derived secondarily from a 2n=4 karyotype. It is a noteworthy finding that active nucleolus organizer (NOR) sites, in terms of silver staining, are tightly linked with the centromere in this species, and that both the centromere and NOR of A₂ were inactivated after the telomere fusion.     

Karyotypic conservation in the mammalian order monotremata (subclass Prototheria)

The order Monotremata, comprising the platypus and two species of echidna (Australian and Nuigini) is the only extant representative of the mammalian subclass Prototheria, which diverged from subclass Theria (marsupials and placental mammals) 150–200 million years ago. The 2n=63, 64 karyotype (newly described here) of the Nuigini echidna is almost identical in morphology and G-band pattern to that of the Australian echidna, from which it diverged about a million years ago. The karyotype of the platypus (2n=52) has several features in common with those of the echidna species; six pairs of large autosomes, many pairs of small (but not micro-) chromosomes, and a series of small unpaired chromosomes which form a multivalent at meiosis. Comparison of the G-band patterns of platypus and echidna autosomes reveals considerable homology. Chromomycin banding demonstrates GC-rich heterochromatin at the centromeres of many platypus and echidna chromosomes, and at the nucleolar organizing regions; some of this heterochromatin C-bands weakly in platypus (but not echidna) spreads. Late replication banding patterns resemble G-banding patterns and confirm the homologies between the species. Striking heteromorphism between chromosomes of some of the large autosomal pairs can be accounted for in the echidna by differences in amount of chromomycin-bright, late replicating heterochromatin. The sex chromosomes in all three species also bear striking homology, despite the difference in sex determination mechanism between platypus (XX/XY) and the echidna species (X₁X₁X₂X₂/X₁X₂Y). The platypus X and echidna X₁ each represent about 5.8% of haploid chromosome length, and are G-band identical. Y chromosomes are similar between species, and are largely homologous to the X (or X₁).     

Leggadas (Mus sp.) de Moundou (Tchad). Observation d'un caryotype aberrant chez une femelle

Pigmy mice (Mus sp.) from Moundou (Tchad), Aberrant karyotype in a female.In a sample of 12 pigmy-mice, one male had the karyotype 2n=36. N.F.=36, sex chromosomes PR, already known from five different taxa, but morphologically it does not belong to any of these. Ten specimens (7 , 3 ) present the karyotype previously described in a population from Ippy (Central African Republic) and for a male from Senegal: , 2n=19, sex chromosomes X/Y₁Y₂; , 2n=18, X/X. Probably we are dealing with a taxonomic entity derived from the superspecies minutoides/musculoides.An aberrant karyotype was found in one female: 2n=19, N.F.=37. The supernumerary is a small acrocentric, morphologically similar to the Y₂ of the male. Its origin could be explained through non-disjunction during the first moiotic division of a paternal spermatocyte, X and Y₃ having passed together to the same daughter cell. If Y₂ is homologous with a segment of the autosomal arm of the X/TR, the complement XXY₂ does not imply the intersexuality of its bearer, but should be considered as an autosomal duplication.     

Cytogenetics of the parthenogenetic grasshopper Warramaba (formerly Moraba) virgo and its bisexual relatives

The distribution of late-replicating segments along the chromosomes of five clones of W. virgo is described. Some, but not all of these segments correspond to C-bands. In general, the autoradiographic profiles (histograms of linear grain density along the length of chromosomes labeled with tritiated thymidine in late S-phase) show strong resemblances throughout the five clones. However, some significant differences exist, and these are particularly marked in the case of the Boulder clone, which is anomalous in many other respects. — A similar study has also been carried out on the two bisexual species of Warramaba (P169 and P196) that gave rise, by hybridization more than half a million years ago, to the parthenogenetic W. virgo. In the case of P169, the autoradiographic profiles of the three large chromosomes (X+A, B+5, CD) which it has contributed to the W. virgo karyotype are extremely similar to those of the corresponding chromosomes in the virgo clones we have studied. In the case of P196 there is likewise, in most instances, a close resemblance of the autoradiographic profiles of the AB, X₁ and CD chromosomes to those of the same chromosomes in the virgo clones, but that of the X₁ shows no particular resemblance to the anomalous profile of the X₁ in the Boulder clone, in which the X₁ has undergone a structural reorganisation. The autoradiographic profile of the P196 CD chromosome does, however, show a much closer resemblance to that of the corresponding chromosome in the Boulder clone than to those of the CD₁₉₆ in the other four virgo clones studied. These investigations confirm the considerable evolutionary stability of DNA replication patterns.     

Morphological and chromosomal characterization of three new continuous cell lines of Drosophila melanogaster

Three continuous cell lines (GM₁, GM₂ and GM₃) were obtained from embryos of Drosophila melanogaster. Karyotypic analysis revealed characteristics distinguishing each line. Except for some minor variations GM₁ cells had an X and a centric heterochromatic fragment (which is a portion of the Y). GM₂ line was characterized by XO cells showing two new telocentric chromosomes while an autosome of the II pair was missing. GM₃ cells were XY; the Y chromosome, however, was shorter than the normal, having a deletion of the terminal section of the short arm. Several problems concerning the origin of these different genomes are discussed.This work was supported by a grant of the Consiglio Nazionale delle Ricerche, Roma.     

Heteromorphic sex chromosomes of lizardfish (Synodontidae): focus on the ZZ-ZW1W2 system in Trachinocephalus myops

The karyotype and DNA content of four lizardfish species (family Synodontidae), that is, Saurida elongata, Synodus ulae, Synodus hoshinonis and Trachinocephalus myops, were analyzed. The karyotype of T. myops significantly differed from that of the other three species having diploid chromosome number of 48 with mainly acrocentric chromosomes and the ZZ-ZW sex chromosome system. The chromosome number of male T. myops was 2n=26, while that of female T. myops was 2n=27. The karyotype consisted of 11 pairs of metacentrics, one pair of acrocentrics and, in addition, two large metacentrics in the male and a single large metacentric, a distinctly small subtelocentric and a microchromosome in the female. C-banding demonstrated that in the female the subtelocentric chromosome and the microchromosome were heterochromatic. The karyotype of T. myops was thought to be derived from a 48 chromosome type synodontid fish through the involvement of Robertsonian rearrangement; the rearrangement of the sex chromosomes proceeded during karyotype evolution. Among the chromosomes, the large metacentrics were determined to be neo-Z (a fusion of the original Z and an autosome), the microchromosomes the W₁ (originally W), and the subtelocentric chromosomes the W₂ (derived from an autosome pair). The miniaturization of W₁ and W₂ chromosomes and their heterochromatinization suggested that sex chromosomes in this species have been already highly differentiated. The findings on DNA content implied that the karyotype of T. myops evolved by centric fusion events without loss in DNA amount.     

Compound kinetochores of the Indian muntjac

The chromosomes of the Indian muntjac (Muntiacus muntjak vaginalis) are unique among mammals due to their low diploid number (2N=6, 7) and large size. It has been proposed that the karyotype of this small Asiatic deer evolved from a related deer the Chinese muntjac (Muntiacus reevesi) with a diploid chromosome number of 2n= 46 consisting of small telocentric chromosomes. In this study we utilized a kinetochore-specific antiserum derived from human patients with the autoimmune disease scleroderma CREST as an immunofluorescent probe to examine kinetochores of the two muntjac species. Since CREST antiserum binds to kinetochores of mitotic chromosomes as well as prekinetochores in interphase nuclei, it was possible to identify and compare kinetochore morphology throughout the cell cycle. Our observations indicated that the kinetochores of the Indian muntjac are composed of a linear beadlike array of smaller subunits that become revealed during interphase. The kinetochores of the Chinese muntjac consisted of minute fluorescent dots located at the tips of the 46 telocentric chromosomes. During interphase, however, the kinetochores of the Chinese muntjac clustered into small aggregates reminiscent of the beadlike arrays seen in the Indian muntjac. Morphometric measurements of fluorescence indicated an equivalent amount of stained material in the two species. Our observations indicate that the kinetochores of the Indian muntjac are compound structures composed of linear arrays of smaller units the size of the individual kinetochores seen on metaphase chromosomes of the Chinese muntjac. Our study supports the notion that the kinetochores of the Indian muntjac evolved by linear fusion of unit kinetochores of the Chinese muntjac. Moreover, it is concluded that the evolution of compound kinetochores may have been facilitated by the nonrandom aggregation of interphase kinetochores in the nuclei of the ancestral species.     

Cytogenetics of the parthenogenetic grasshopper Moraba virgo and its bisexual relatives

An undescribed bisexual grasshopper species closely related to the all-female parthenogenetic Moraba virgo has an X₁X₂Y sex chromosome mechanism which incorporates 7 chromosomes of the ancestral karyotype (the original X and three pairs of autosomes). Apparently three separate chromosomal fusions have occurred, one of these being a tandem fusion followed by a crossover which stabilized it in the sex chromosome system. M. virgo probably arose from an ancestor which had X₁X₂Y males but lacked the tandem fusion present in the new species, the females of virgo having the constitution X₁X₁X₂O.Supported by Public Health Service Grant GM-07212 from the Division of General Medical Sciences, U.S. National Institutes of Health, and by a grant from the Australian Research Grants Committee.     

Karyological analysis of Cylindera trisignata (Latreille & Dejean, 1822) from Portugal (Coleoptera,Cicindelidae)

Cellular suspensions of germinal tissues of Cylindera trisignata provided the definition of its karyotype: 2n=23 and 2n=26 for the males and females respectively. This Palearctic species has a sex chromosome system of the X₁X₂X₃X₄Y / X₁X₁X₂X₂X₃X₃X₄X₄ type, only found until now in Cicindela maroccana pseudomaroccana. The heterosomes are not well-differentiated from the general morphological standpoint. To explain the origin of the 4X condition, a mechanism of dissociation of the X chromosomes rather than an incorporation of autosomal segments is proposed. However, based on the occurrence of distinct ploidy levels, both in male and female cells, with a relatively high incidence for the tetraploid condition with regular autosomal bivalents, the hypothesis of a possible evolutionary role of polyploidy is suggested.     

Chromosome banding in Amphibia

The karyotype of the marsupial frog Gastrotheca riobambae is characterized by exceptionally highly differentiated XY/XX sex chromosomes. The 18S and 28S ribosomal RNA genes were found only in the nucleolus organizer region (NOR) of the X chromosome by in situ hybridization, silver staining and mithramycin banding. This amphibian species therefore exhibits a sex-specific difference in the number of ribosomal RNA genes of about 2()1(). This constitutes an extremely rare situation in the karyotype of vertebrates. Examination of various somatic tissues from female animals showed that the NORs on both X chromosomes are always active. The results are discussed in relation to the apparent absence of dosage compensation for sexlinked genes in the Amphibia.     

New insights into sex chromosome evolution in anole lizards (Reptilia,Dactyloidae)

Anoles are a clade of iguanian lizards that underwent an extensive radiation between 125 and 65 million years ago. Their karyotypes show wide variation in diploid number spanning from 26 (Anolis evermanni) to 44 (A. insolitus). This chromosomal variation involves their sex chromosomes, ranging from simple systems (XX/XY), with heterochromosomes represented by either micro- or macrochromosomes, to multiple systems (X₁X₁X₂X₂/X₁X₂Y). Here, for the first time, the homology relationships of sex chromosomes have been investigated in nine anole lizards at the whole chromosome level. Cross-species chromosome painting using sex chromosome paints from A. carolinensis, Ctenonotus pogus and Norops sagrei and gene mapping of X-linked genes demonstrated that the anole ancestral sex chromosome system constituted by microchromosomes is retained in all the species with the ancestral karyotype (2n?=?36, 12 macro- and 24 microchromosomes). On the contrary, species with a derived karyotype, namely those belonging to genera Ctenonotus and Norops, show a series of rearrangements (fusions/fissions) involving autosomes/microchromosomes that led to the formation of their current sex chromosome systems. These results demonstrate that different autosomes were involved in translocations with sex chromosomes in closely related lineages of anole lizards and that several sequential microautosome/sex chromosome fusions lead to a remarkable increase in size of Norops sagrei sex chromosomes.     

The karyotype of some Australian species of Macropathinae (Gryllacridoidea — Rhaphidophoridae)

The chromosome number of eight species of Australian Macropathinae are reported in this paper. They show basically the same karyotype as four South-American species of the genus Heteromallus. The basic karyotype for the subfamily Macropathinae is established as 2n=45-and 46, with an XO-XX sex mechanism; the X is metacentric.The genus Cavernotettix comprises a cytologically modern and dynamic group of species in which the basic karyotype is modified in several directions. There are two species with XY-XX sex mechanisms and three with X₁X₂YX₁X₁X₂X₂ systems. The latter mechanisms are reported for the first time in Gryllacridoidea.Sponsored by C.A.P.E.S. (Río de Janeiro, Brasil) and F.A.P.E.S.P. (São Paulo, Brasil).     

Ultrastructural characterization of the sex chromosomes during spermatogenesis of spiders having holocentric chromosomes and a long diffuse stage

An ultrastructural study has been made of spermatogenesis in two species of primitive spiders having holocentric chromosomes (Dysdera crocata, XO and Segestria florentia X₁X₂O). Analysis of the meiotic prophase shows a scarcity or absence of typical leptotene to pachytene stages. Only in D. crocata have synaptonemal complex (SC) remnants been seen, and these occurred in nuclei with an extreme chromatin decondensation. In both species typical early prophase stages have been replaced by nuclei lacking SC and with their chromatin almost completely decondensed, constituting a long and well-defined diffuse stage. Only nucleoli and the condensed sex chromosomes can be identified. — In S. florentina paired non-homologous sex chromosomes lack a junction lamina and thus clearly differ from the sex chromosomes of more evolved spiders with an X₁X₂O male sex determination mechanism. In the same species, sex chromosomes can be recognized during metaphase I due to their special structural details, while in D. crocata the X chromosome is not distinguishable from the autosomes at this stage. — The diffuse stage and particularly the structural characteristics of the sex chromosomes during meiotic prophase are reviewed and discussed in relation to the meiotic process in other arachnid groups.     

Cytogenetics of the chinese giant salamander,Andrias davidianus (Blanchard): the evolutionary significance of cryptobranchoid karyotypes

Cytogenetic aspects of the cryptobranchid salamander Andrias davidianus of western China have been studied, including chromosome number and morphology, C-band patterns, meiosis, and the chromosomal localization of ribosomal 5S RNA genes. Our data regarding chromosome number (2n=60) and general chromosome morphology largely confirm the results of Morescalchi et al. (1977). The karyotype consists of 16 pairs of macrochromosomes that decrease gradually in relative length to 14 pairs of microchromosomes. Telocentric chromosomes are a conspicuous feature of the karyotype, representing more than half the genome. Differential staining reveals that all of the chromosomes, except four pairs of microchromosomes, have C-band heterochromatin in their centromeric regions, the amount varying irrespective of chromosome size. Faint bands of interstitial and telomeric C-band heterochromatin are found in mitotic chromosomes but are not seen in meiotic preparations. In C-banded mitotic preparations from a female, one of the smallest macrochromosome pairs is heteromorphic in respect to C-band heterochromatin and centromere position. In situ hybridization of an iodinated 5S RNA probe to meiotic chromosome preparations reveals that this repeated gene is clustered near the telomeric region of chromosome 7, a medium size telocentric, a location corresponding to a band of heterochromatin. Studies of spermatocytes indicate that the process of meiosis in A. davidianus closely resembles that of more advanced salamanders, and that the microchromosomes are meiotically stable. The significance of microchromosomes and chromosome morphology in the reorganization of salamander genomes during evolution is discussed on the basis of cytogenetic data available for A. davidianus and various other primitive and advanced salamanders.     

Chromosome banding in Amphibia

The chromosomes of the South American marsupial frogs Gastrotheca fissipes, G. ovifera, G. walkeri and Flectonotus pygmaeus were analyzed by means of conventional and various banding techniques. The karyotypes of G. ovifera and G. walkeri are characterized by highly differentiated XY/XX sex chromosomes. Whereas the X chromosomes and autosomes contain large amounts of constitutive heterochromatin, extremely little heterochromatin is located in the Y chromosomes. This is in contrast to all previously known amphibian Y chromosomes and the Y chromosomes of most other vertebrates. In the male meiosis of G. walkeri, the euchromatic segments of the heteromorphic XY chromosomes show the same pairing configuration as the autosomal bivalents. The karyotype of F. pygmaeus is remarkable for the unique presence of telocentric chromosomes and the high frequency of interstitially located chiasmata in the meiotic bivalents. The evolution of the karyotypes and sex chromosomes, the structure of the various classes of heterochromatin and the data obtained from meiotic analyses of the marsupial hylids are discussed.     

Cytogenetic components of reproductive isolation in Trimerotropis thalassica and T. occidentalis

The grasshopper Trimerotropis thalassica (Bruner) has a diploid count of 2n=23 (XO), 24 (XX). The two largest autosomes pairs are regularly metacentric, a consequence of fixed pericentric inversions. The X-chromosome is also a fixed metacentric. The remaining nine pairs of autosomes are polymorphic for floating pericentric inversions so that the complement consists of a mixture of telocentric and metacentric members. Trimerotropis occidentalis (Bruner) is polymorphic for comparable inversions in only two of its autosome pairs and has a telocentric X. It is, however, unique among the species of the genus Trimerotropis in having only 21 chromosomes in its male diploid set in all the populations so far studied. A single male found in a mixed population of these two species at Jasper Ridge, Stanford University, was characterised by a count 2n=22. In both this respect and in its phenotype it was intermediate in character, representing a natural F₁ hybrid between the two species. Cytogenetic analysis of this hybrid male indicated that occidentalis is differentiated from thalassica only in respect of a single tandem translocation. This has involved two of the telocentric elements of thalassica which have fused into a single composite telocentric partly homologous with each of the smaller progenitors. Although potentially capable of forming a multiple of three, one or other of the progenitor chromosomes regularly fails to pair with the tandem product in the hybrid so that one or more univalents invariably occur. These, by lagging, prevent cytokinesis and subsequently lead to the formation of macrospermatids which inevitably produce a measure of sterility. It is argued that this sterility provides a basis for reproductive isolation.In fond memory of my assistant, collaborator and friend Michael George St. Clair-Freeman who died tragically in a motor accident in Canberra on September 16th 1976