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
Summary Meiotic analyses at pachytene, anaphase I, telophase I, prophase II, metaphase II and the tetrad stage were carried out on several Capsicum annuum L. plants. The bivalents showed regular chromosome pairing and a nonrandom chiasmata distribution. The mean number of chiasmata per cell were 21.62. Aberrations such as reverse inversion loops in pachytene, dicentric bridghes and fragments in pollen mother cells at anaphase I, telophase I, prophase II, metaphase II and 4, 5, 6 and 7 microspores at the tetrad stage, indicated that these plants were heterozygous for a paracentric inversion, which may involve 1 or more homologous chromosomes. The 84.04% fertility showed that a simple or double crossing-over took place within reverse inversion loops between inverted and normal chromosome segments at pachytene, which reduced fertility by the formation of genetically abnormal gametes.
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