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.

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

Karyological instability during pollen mother cells formation in Aloe vera (Aloaceae). In order to clarify possible cytogenetic anomalies that reduce pollen fertility, premeiotic mitosis was studied in Aloe vera plants from a naturalized population in the northeast of Venezuela (10°34'15'' N and 64°12'08'' W). Karyological configurations were evaluated during the stage of cell proliferation leading to the formation of pollen mother cells (PMCs). The sampling was carried out in March 2005, choosing inflorescences without mechanical or biological damage from 25 plants selected at random. Flower buds (FB) 2 to 6 mm in length were collected from 7:00 AM through 6:00 PM, their perianths removed, and fixed in Carnoy I (3:1 ethanol/glacial acetic acid) for 24 h and stored in ethanol (70 % v/v) until observation. Light microscope observations were done on temporary preparations obtained by overflowing anther content suspended in acetic orcein (1.5 % w/v) for 5 min and softly squashing with the cover slip. A total of 9 411 cells were analyzed. Upper mitotic activity was observed in FB from 3.8 ± 0.09 mm long, collected at 11:00 AM through 1:00 PM; 17% of PMCs showed one to eight sister chromatid bridges from anaphase to telophase; 13%, one to seven micronucleus of variable diameter (0.9 to 4.8 µm); 8.1% were united by thin chromatin filaments, and 0.1% lacked a nucleus. Other evaluated cells (61.8%) had apparently normal mitotic configurations, without considerable morphometric variations. The evident irregular proliferation of a PMCs fraction (39.2%) suggests that environmental stress conditions (day temperatures ranging 32.7 to 39.8ºC, high solar radiation and low humidity) induce chromosome instability and physiologic changes that affect the normal development during premeiotic mitosis. As a consequence, loss or addition of chromosome fragments can occur in association with deficiencies and gene duplications. Rev. Biol.

Summary Cytogenetic traits were examined in Aloe saponaria (2S), A. vera (2V and 4V), and the experimental diploid (VS) and triploid (VVS) hybrids. Karyotype 2nϭ2xϭ14 (8Lϩ6S) was confirmed in 2S and 2V, whereas 2nϭ4xϭ28 (16Lϩ12S) was ascribed to 4V. VS showed the expected karyotype 2nϭ2xϭ14 (8Lϩ6S), with slight differences between the long (L) homologues. VVS also showed the expected karyotype 2nϭ3xϭ21 (12Lϩ9S); however, several chromosome deviations reflecting the absence or addition of small (S) chromosomes, terminal deletions of variable size which led to the formation of atypical chromosomes, or the loss of a L chromosome were also observed. The implication of these karyological variations is discussed.

Reproductive phenology and flower anatomy of the plants Aloe vera and Aloe saponaria (Aloaceae) in Cumana, Venezuela. The reproductive phenology and the floral anatomy of two species of Aloe kept under nursery conditions were analized in eastern Venezuela from September 2001 to September 2002. A. vera flowered between December and May; A. saponaria from November through August. In both species, the anthesis lasted 48 h., the anther dehiscence coincided with the opening of the perianth, the stigma receptivity was higher on the second day of anthesis and nectar production reached 0.34 ml/flower. In A. vera 228±77 yellow flowers on long scapes of 76±11 cm and with 1-3 shafts were observed; A. saponaria had 94 ± 33 orange flowers on more compact scapes with 3-5 shafts. The pollen/ovule ratio was 4 115.2 in A. vera and 3 247.1 in A. saponaria, thus they can be classified as compulsory xenogams. Frequent visits of Apis mellifera, Trigona sp., Poliste sp., Eumenes sp., Vespa sp., Leucippus fallax and Amazilia tobaci were recorded. Although both species showed a high production of pollen and nectar, and the presence of potential pollinators, by the end of the flowering period, fruits were only observed in A. saponaria, with a 12 % reproductive efficiency. Coupled with the stigmatic receptivity results and the preliminary intra and interspecific crossing experiments, this suggests the existence of protandry and self-incompatibility as reproductive barriers reducing endogamy in these species.