Browsing by Author "Dessalegn, Fikre"

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    A - Comparative Study on the Population Structure, Reproductive Biology and Chromosome Cytology of Two Endemic Aloe Species ( Aloe Pulcherr1ma Sebsebe and Gilbert and Aloe Deep Ana Christian) in Ethiopia
    (Addis Ababa Universty, 1999-06) Dessalegn, Fikre; Demissew, Sebsebe (Professor)
    A study was carried out to investigate and compare the population structure, reproductive biology and chromosome cytology of two endemic Aloe species, A. pulcherrima (with rare occurence) and A. debrcma (common) on naturally occurring populations around Debre Libanos in central Ethiopia from May 1997 to February 1999. The purpose of the study was to find out the causes for the rare occurrence of A. pulcherrima in comparison to the relative abundance of the sympatric species, A. debrcma with the aim of developing appropriate conservation strategies to conserve these endemic plants. Six plots of (5 x 20 m2) were established at the study site; three of these include the A. pulcherrima and the other three include A. debrcma populations. In each plot, genets and ramets were sorted out and the population structure was described by clone size and rosette diameter of ramets. Size of ramet relationship to flowering and size of ramet and flowering relationship to growth rate were studied. In addition, population flux (dynamics) at genet and ramet level was calculated and compared for the two species. Reproductive success and dispersal efficiency in the two species were estimated and compared based on the extent of vegetative propagation; production of flowers, fruits, seeds and also by pollinator availability and percentage pollen viability in the populations studied. Chromosome cytological studies, i.e., number, pairing and segregation of meiotic chromosomes were conducted in the laboratory from flower buds at appropriate stages of development and comparison made. Environmental factors which influence the development and distribution of the Aloes such as soil, climate, vegetation, herbivory and human oriented activities associated in the study site were obtained and analyzed. The results of the study revealed that A. pulcherrima has very old stagnant “ senile” population with no juvenilization (no recruitment from seed) but entirely depend on vegetative propagation and spread in spite of good seed set, satisfactory pollinatory activity (Sun birds and Bees) and high percentage pollen viability. In addition, it is attacked by a rust fungus parasite (Uromyces aloes) and heavily grazed by goats and baboons. A. debrana on the otherhand has a more viable population with adequate recruitment both from vegetative propagation and sexual reproduction. Moreover, the production of flowers, fruits and seeds were found to be greater than in A. pulcherrima. As in A. pulcherrima, it has satisfactory pollinatory activity (sun birds and bees) and high percentage pollen viability. A. debrana appears to be “ buffered” from rust fungus attack, less often eaten by goats and baboons, and is expanding. The two species also differ in their flower phenology; A. pulcherrima flowering from June to August and A. debrana flowering from December to February. Thus, this avoids competition for pollinators and hybridization between them. Both species also show low mortality at both ramet and genet level. Chromosome cytological study showed that the two species have diploid chromosome number 2n=14; 8 large and 6 small sized. Meiotic activities from pollen mother cells (PMCs) were also found normal with 7 bivalent (711) formation at metaphase I and absence of aberrations such as laggards (univalents and/or separated chromatids), bridges and fragments at anaphase/telophase I and II that ultimately end up in micronuclei formation. These were also evidenced by high percentage pollen viability in the anthers of the species studied. Thus with the need to protect these endemic species, appropriate conservation strategies have to be suggested.