Karyotypes of three species of toadfish (Amphichthys cryptocentrus, Batrachoides manglae and Thalassophryne maculosa), reported here for the first time revealed diploid complements of 46 chromosomes. Karyo-evolutionary trend of these species is discussed. Based on differences in arm number, we suggest that T. maculosa possesses the most evolved karyotype among the three species, hence it would be the more recent one, followed by B. manglae and A. cryptocentrus.
Summary Autotetraploid plants of Aloe vera L. were induced by colchicine treatment. Diploid shoot tips (2n =2x=14) were treated by immersion of the rhizomes in colchicine concentrations of 0.05%, 0.10% and 0.15% at exposure times of 6, 12, 18 and 24 h. The highest proportion of polyploid plants (5.9%) with 2n=4x=28 was obtained from a 0.15% colchicine and 24 h combination. Diploid/tetraploid chimerical plants were observed at the lowest colchicine concentration/time treatments. Expression of gigantism in morphological and cytological characters, such as, width and thickness of the leaves, size and frequency of juvenile foliar spots, plant weight, size of epidermal and root cells, were noted in autotetraploid plants with a significant decrease in stomatal frequency.
Karyotypes of Mugil liza and M. curema from Venezuela were studied. M. liza karyotype is 2n=48 acrocentric chromosomes, but M. curema is 2n=24, containing one submetacentric and 11 metacentric pairs. The chromosomic complement described for M. liza constitutes the first report for the speciesand coincides with the modal karyotype (2n=48) in the Mugilidae family. Venezuelan M. curema karyotype differs in number and shape from M. curema from Louisiana (USA), which has been reported to have a complement 2n=28 (10 metacentric pairs, two subtelocentric pairs and two acrocentric pairs).
Summary Meiotic analyses at pachytene, anaphase I and II, metaphase II and the tetrad stage were carried out on several Tridax procumbens L. plants. Aberrations such as reverse inversion loops in pachytene, dicentric bridges and (or) fragments in pollen mother cells at anaphase I and II, dicentric bridges at metaphase II and 4-7 microspores at the tetrad stage, indicated that these plants were heterozygous for a paracentric inversion, which may involve one or more homologous chromosomes. The 75% fertility showed that a simple or double crossing-over took place within inversion reverse loops between inverted and normal chromosome segments at pachytene, which reduced fertility by the formation of genetically abnormal gametes. Key words Paracentric inversions, Fertility, Meiosis, Chromosomes, Crossing-over.Tridax procumbens L. is a perennial herb, widely distributed in Mexico, The Antilles and South America. In Venezuela, it is common on roadsides and in vacant lots in towns (Aristeguieta 1964). Cytological studies have shown that 2nϭ36 (Raghavan and Ventakasubban 1941). In a paracentric inversion, if simple or double crossing-over takes place within inversion reverse loops between inverted and normal chromosome segments at pachytene, dicentric bridges and acentric fragments in some pollen mother cells (PMCs) at meiotic anaphase I and II, are observed (McClintock 1938), which reduces fertility by the formation of genetically abnormal gametes. On the other hand, if no crossovers take place, then only normal PMCs are visible at anaphase. The meiotic configurations expected at anaphase following crossovers within the inversion loop and proximal to it, have been reported by McClintock (1938), Brandham (1969), Ekberg (1974), Sybenga (1975 and Pickering (1991).This study reports different meiotic configurations at pachytene, anaphase I and II, telophase I and II, suggesting that crossing-over occurs within inversion reverse loops in a paracentric inversion of a T. procumbens population. Materials and methodsTridax procumbens seeds used in this study were collected from a naturalized population in Cumaná, Sucre State, Venezuela during the year 2000. Selected seeds from 5 plants were germinated in 50% Hoagland's solution. Seedlings approximately 7 cm long were planted in water expanded Jiffy-7 peat pellets until numerous roots were observed. Seven week-old seedlings were transferred to 5 kg polyethylene sacks containing sand, peat and soil in a 1 : 1 : 1 ratio and maintained in a greenhouse.Flower buds were harvested between 13 : 00 and 13 : 30 and fixed in ethanol: propionic acid
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