Evolution of the Proto Sex-Chromosome in Solea senegalensis

Solea senegalensis is a flatfish belonging to the Soleidae family within the Pleuronectiformes order. It has a karyotype of 2n = 42 (FN = 60; 6M + 4 SM + 8 St + 24 T) and a XX/XY system. The first pair of metacentric chromosomes has been proposed as a proto sex-chromosome originated by a Robertsonian fusion between acrocentric chromosomes. In order to elucidate a possible evolutionary origin of this chromosome 1, studies of genomic synteny were carried out with eight fish species. A total of 88 genes annotated within of 14 BACs located in the chromosome 1 of S. senegalensis were used to elaborate syntenic maps. Six BACs (BAC5K5, BAC52C17, BAC53B20, BAC84K7, BAC56H24, and BAC48P7) were distributed in, at least, 5 chromosomes in the species studied, and a group of four genes from BAC53B20 (grsf1, rufy3, slc4a4 and npffr2) and genes from BAC48K7 (dmrt2, dmrt3, dmrt1, c9orf117, kank1 and fbp1) formed a conserved cluster in all species. The analysis of repetitive sequences showed that the number of retroelements and simple repeat per BAC showed its highest value in the subcentromeric region where 53B20, 16E16 and 48K7 BACs were localized. This region contains all the dmrt genes, which are associated with sex determination in some species. In addition, the presence of a satellite “chromosome Y” (motif length: 860 bp) was detected in this region. These findings allowed to trace an evolutionary trend for the large metacentric chromosome of S. senegalensis, throughout different rearrangements, which could be at an initial phase of differentiation as sex chromosome.

Karyotypic Analysis of four Species of Genus Blumea (Asteraceae) from Nepal

In this study chromosome number determination and karyotype analysis of four species of genus Blumea from the family asteraceae was carried out. The specimen plants were collected from central parts of Nepal, namely Blumea fistulosa (Roxb.) Kurz, Blumea lacera var.glandulosa (DC.) Hook, Blumea lacera (Buem f.) DC. and Blumea laciniata DC were observed. The chromosome number in somatic cells were recorded to be 2n= 22 in Blumea fistulosa; 2n= 32 in Blumea lacera var.glandulosa; 2n=18 in Blumea lacera and Blumea laciniata. The range of chromosome length found to be 0.6 to 1.6 µm in Blumea fistulosa, 0.6 to 1.6µm in Blumea lacera var.glandulosa, 0.6 to 1.7 µm in Blumea lacera and 0.8 to 1.6 µm in Blumea laciniata. Karyotype formula for Blumea fistulosa is M12+ sm10, for Blumea lacera var. glandulosa is M14+sm14+ st4, for Blumea lacera is M14+ st2 and for Blumea laciniata is M12+ sm6 in Blumea laciniata. In this investigation a pair of satellite chromosome found in only one species Blumea fistulosa at the end of shot arm of chromosome. Mainly three types of chromosomes observed in this study having centromere at middle point, at sub-median region and at sub-terminal region. Numerical and structural variation in chromosome are evolutionary significance. Similarity in size of chromosomes and karyomorphology indicates the homogeneity of the taxa within this tribe. Int. J. Appl. Sci. Biotechnol. Vol 6(2): 115-121

Genome size, heterochromatin organisation, and ribosomal gene mapping in four species of Ribes

Four wild Ribes species (Ribes alpinum L., Ribes petraeum Wulf., Ribes rubrum L., and Ribes uva- crispa L.; all 2n = 2x = 16) were surveyed for their chromosome and genome organisation. Their genome size was assessed using flow cytometry. Ribes alpinum had 5.3% more nuclear DNA than did the three other species, whose average was 2C = 1.91 pg with 40.4% GC. In addition, GC- and AT-rich heterochromatin and rDNA (18S–5.8S–26S and 5S) patterns were studied using fluorochrome banding and double-target fluorescence in situ hybridization (FISH), respectively. Only GC-rich heterochromatin was detected, co-localizing with 18S–26S rDNA. Fluorochrome banding and FISH patterns revealed marked differences between species. Ribes alpinum and R. uva-crispa differed from R. rubrum and R. petraeum by the number of 18S–26S sites and the localization of 5S rDNA. Ribes alpinum and R. uva-crispa were differentiated by the number of 5S sites. Ribes rubrum and R. petraeum also differed by the number of 5S sites and by the size of the GC-rich band on the satellite chromosome pair. These results should contribute to a better understanding of phylogenetic relationships among these species.Key words: Ribes, flow cytometry, fluorochrome banding, FISH, rDNA, NORs.

A CYTOGENETIC STUDY IN ALLIUM FISTULOSUM, A. CEPA, THEIR F1 HYBRID, AND BACKCROSS PROGENY

Cytogenetic studies were performed on Allium fistulosum, A. cepa. their F1 hybrid, and ten backcross (BC1) progenies [(A. fistulosum × A. cepa) × (A. cepa) 1. In meiosis the F1 hybrid showed 41 percent hetercmorphic bivalent pairing with 10.6 +. 1.8 `chiasmata per cell. Meiocytes were observed with one, two and three bridges and fragments, indicating at least three paracentric inversions. Multivalent associations indicate at least two translocations, one involving the satellite chromosome. The percentage of bivalent pairing, bridges and fragments, and multivalent associations varied in BC1 progenies. The F1 hybrid and all of the BC1 plants were either sterile or had very little seed set. The satellite chromosomes used as cytological markers showed variation in nucleolus position, degree of attachment and number.

Polysomy and supernumerary chromosomes in Ornithogalum umbellatum L. (Liliaceae)

One hundred bulbs of Ornithogalum umbellatum L. have been analyzed cytogenetically in one natural population collected from the Sierra Nevada of Spain. Three types of plants were identified: (i) diploid plants (2n = 18, 26%); (ii) diploid plants with a variable number of B chromosomes (1–8 B's, 40%); and (iii) polysomic plants (2n = 19–23) with or without B's (34%). B's are of two types: metacentric and acrocentric and are associated with three types of abnormalities: (i) failure of the A chromosomes to move to the poles at anaphase I; (ii) nondisjunction of some A chromosomes at anaphase II; and (iii) the occurrence of tetraploid sporocytes. The B's are isopycnotic, do not associate with the A's, and invariably occur as univalents at first meiosis but show no tendency for elimination. All members of the complement except the satellite chromosome, no. 6, have been detected in a trisomic or a tetrasomic condition. Additionally some unusual structural variants, not present in the diploid standard complement, appear in some polysomic individuals. The polysomic elements are euchromatic, stable, and they do not associate either with normal chromosomes or with B's. The polysomic elements form univalents when they are trisomic and bivalents when tetrasomic. The fact that a high frequency (94%) of the polysomic elements also carry B's suggests that these two forms of numerical variation are interrelated in origin. Key words: Ornithogalum umbellatum, polysomy, supernumerary chromosomes.

Interchange polymorphism in natural populations of Allium paniculatum L. (Liliaceae): nature, frequency, effects, and mechanism of maintenance

A reciprocal translocation was found with a high frequency (average 44.44%) in four natural populations of Allium paniculatum L. (Liliaceae) from the South of Spain. The chromosomes involved are 1 and 7. The translocation is reciprocal and unequal. Chiasma frequency in the chromosomes not involved in the interchange is not affected, but chiasma frequency is decreased in the translocated chromosomes in the heterozygotes. As a satellite chromosome is involved in the interchange, the nucleolus is associated with the quadrivalent and the pattern of nucleolus formation is changed in heterozygotes, which have a lower mean number of nucleoli [Formula: see text] than homozygous standard individuals [Formula: see text]. The spontaneous mutation rate for interchanges during the early stages of microsporogenesis is high (μ = 1.08 × 10−2). No interchange homozygotes were found in any of the four populations analyzed. Furthermore, a comparative analysis of heterozygous (HT) and homozygous standard (HM) individuals in two populations demonstrated that homozygous standard plants show, on the whole, higher fitness than the heterozygotes. This can be attributed to a greater egg cell fertility and seed set. The possible causes of maintenance are discussed: the interchange in A. paniculatum is probably not maintained by overdominance for generative reproductive characters, nor by a mutation–selection equilibrium. One possibility, that heterozygotes have superior vegetative reproduction, still remains open for future investigation.Key words: Allium paniculatum, interchange polymorphism, fitness.

Alien chromosome fragments conditioning resistance to beet cyst nematode in diploid descendants from monosomic additions of Beta procumbens to B. vulgaris

A monosomic addition plant of Beta vulgaris, containing an extra chromosome of B. procumbens with a gene for resistance to the beet cyst nematode, gave rise to a diploid descendant with an additional chromosome fragment. This fragment was found to bear the gene for resistance. A study of its morphology and behaviour made it likely that it represents the short arm of the alien chromosome and may have originated by centric fission of the single addition chromosome at anaphase I or II. The fragment showed incomplete generative transmission. In part of the somatic cells the fragment was missing and occasionally more than one fragment per cell was observed. The transmission of resistance in the telosome addition material was about 10%, which is 2.5 times lower than that of the monosomic addition. The pattern of transmission can be explained by an incomplete centromere, otherwise disturbed centric activity, or chromatid cohesiveness. Comparison with other monosomic addition plants showed that the resistance gene is located on the short arm of the satellite chromosome of B. procumbens and thus is linked to the nucleoar organizer region. The long arm of this chromosome possesses genes influencing annuality and leaf shape.Key words: Beta vulgaris, Beta procumbens, beet cyst nematode, monosomic addition, centric chromosome fragment.

CHROMOSOME BREAKAGE INDUCED IN VICIA FABA ROOT TIPS BY 2,4,6-TRI(ETHYLENEIMINO)-1,3,5-TRIAZINE

After a 12-hour treatment in 10−5 M solution of 2,4,6-tri(ethyleneimino)-1,3,5-triazine (TEM), up to 29% of Vicia and 31% of Allium root cells in anaphase had bridges or fragments. The most abundant metaphase aberrations were chromosome breaks and chromatid interchanges. Aberrations were more numerous after 6-hour treatment with 2 × 10−5 M than after 12-hour treatment with 10−5 M solution. The peak aberration frequencies were reached at 36 hours after 2 × 10−5 M treatments and slightly earlier with weaker solutions. Many chromosomes had incomplete breaks. Sister reunions of chromatids occurred with equal frequency in centric and acentric fragments. Sister reunion was 1.5 times as frequent as chromatid exchange. Along the length of the satellite chromosome, breaks occurred at random except for a slight favoring of the heterochromatic region. Per unit of length the short chromosomes were about 3 times as susceptible to breakage and exchange as satellite chromosomes. TEM introduced through cut stems had no discernible effects on Tradescantia microspore chromosomes even after 3 to 5 days of treatment.