Cytogenetic Study of Paracentric Inversions in Tridax procumbens (Compositae)

Abstract

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

Induced Physical Mutagenesis and its Effect in Cytological Behavior of Ashwagandha (<i>Withania somnifera</i> (L.) Dunal)

“…Bridge formation observed by (Ahmad and Yasmin 1992), (Utsunomiya et al, 2002) and (Cequea et al, 2003). Bridges might have occurred as a result of delayed terminalization (Bipasha and Shella, 1992), unequal separation of chromosomes .…”

“…The Ashwagandha seeds were treated with (5,10,15,20,25,30,35,40, 45 and 50KR) of gamma rays. The untreated seeds were used as control.…”

“…The plant has tremendous medicinal values and it is known from ancient times. The dry and well matured seeds of ashwagandha were irradiated with different doses of gamma rays viz., 5,10,15,20,25,30,35, 40, 45 and 50 KR respectively. The chromosome number of control plant is 2n = 48.…”

Morphological and cytogenetic characterization in experimental hybrid Aloe jucunda Reyn. x Aloe vera (L.) Burm. f. (Asphodelaceae)

“…Chromosomal fragments present individually or linked to dicentric bridges during anaphase-I or telophase-I generally form additional micronuclei that increase the number of microspores at the end of meiosis and cause gene defi ciencies (Ahirwar and Verma 2013). Th ese aber-rations may be present in clones that were formed by vegetative propagation from carrier individuals (populations of A. vera from Eastern Venezuela) and may have been inherited by the progeny or may be generated during the formation of sexual cells (Cequea et al 2003, Imery 2011. On the other hand, Baptista et al (2000) mention that the high frequency of sticky chromosomes or agglutination could be due to a genetic-environmental interaction associated with high temperatures, causing chromatin instability mainly in metaphase-I.…”

Inestabilidad cariológica durante la formación de células madres del polen en Aloe vera (Aloaceae)

“…Algunos autores atribuyen el origen de las anormalidades premeióticas a daños mecánicos o heridas cercanas a la yema floral (Sarvella 1958), errores de fijación y manipulación de las muestras (Takats 1959, Kamra 1960, ataque de patógenos (Morisset 1978), contaminación con productos químicos de uso agrícola (Bobak y Herich 1978) y mutaciones puntuales en los genes que regulan los mecanismos de sinapsis, entrecruzamiento y separación de cromátidas (Omara 1976). En el caso particular de A. vera, la selección sistemática de botones florales en condiciones óptimas de desarrollo y la aplicación de un protocolo estándar de preparación (Imery 2002, Cequea et al 2003, reducen los errores experimentales y respaldan la posibilidad de que las anormalidades premeióticas observadas tengan un origen genético o resulten de una considerable sensibilidad ambiental.…”