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Systematics and Genetics of the Leucaena diversifolia (Schlecht.) Benth. Complex
|Title:||Systematics and Genetics of the Leucaena diversifolia (Schlecht.) Benth. Complex|
|Authors:||Pan, Fuh Jiunn|
|Abstract:||The Leucaena diversifolia complex has thirteen described species which are found mainly in southern Mexico and northern Central America. Living plants were cultivated at Waimanalo Research Station, Hawaii. The present study, involving morphological, cytological and isozymic examinations, and an extensive program of hybridization, including genetic study of F1 and F2 hybrid progenies, makes it possible for the first time to clarify taxonomic relationships within.|
Cytological investigation showed that two gametic chromosome numbers occur within the complex. Diploid counts with n = 26 were obtained for 15 accessions. Fourteen accessions were found to be tetraploids with n = 52, among which four accessions were morphologically different from other accessions. All accessions of the complex observed frequently carried 1 to 8 extra chromosomes which may be B-chromosomes. Gametic chromosome numbers of 8 other species of Leucanea were also presented. One species, L. leucocephala, was confirmed as tetraploid (n = 52). Numbers of n= 26 were observed for L. collinsii, L. lanceolata, L. macophylla, L. shannoni and L. trichodes, and n = 28 for L. pulverulenta and L. retusa. The F1 chromosome behavior of sesven hybrid combinations involving six taxa revealed that 26 chromosomes of all the diploid species examined were highly homologous to each other, and to half the genome of each of the tetreaploid species.
Study of herbarium specimens indicated that the Leucaena diversifolia complex could be divided into three groups. Diagnostic pubescence on leaflets and stem tips attributed the complex were shown to variate even within a single accession. An investigation of 30 accessions of living plants, utilizing 8 qualitative and 5 quantitative characteristics also revealed the presence of three groups in the complex: diploid L. diyersifolia (DIV2N), tetraploid PSL. diversifoliaPS (DIV4N) and L. pallida (PAL). Morphological features serving to distinguish these groups were corola/calyx length ratio, lengths of stamens and styles, and inflorescence (head) size. DIV2N was found to be highly variable in most characters observed. Tetraploids occurred only in Veracruz, Mexico whereas diploids had much wider areas of distribution occurring from southern Mexico to northern Nicaragua. They were allopatric. Pollen size and size of stomatal guard cells were correlated with ploidy level of L. diversifolia.
The self-pollination program revealed that diploid Leucaena diversifolia and L. pallida were self-compatible while tetraploid L. diversifolia were self-compatible. Analysis of F1 progeny in diploid L. diversifolia indicated that the self-incompatibility was of the gametic type. The crossability of different groups within the complex was determined. There was no indication of any genetic crossing barrier between different groups, and even between morphologically different species. The F1 hybrids of both intra-specific and inter-specific crosses were remarkably vigorous. The F2 hybrids resulting from F1 sib-crossing of diploid L. diversifolia and two other diploid species, L. lanceolata and L. shannoni, however, showed genetic breakdown. Controlled pollination using F1, F2 and backcross hybrids of two different accessions of L. diversifolia showed single gene Mendelian inheritance of vesture on leaflets, rachilla and stem tips, with the pubescence alele dominant over the glabrous allele. Genetics of leaflet pairs per pinna and petiolar gland type were also discussed. Two cytotypes of L. diyersifolia were separated by strong sterility barriers.
A study of peroxidase polymophism in different tissues and ages of Leucaena leucocephala was presented. Some peroxidase isozymes and their intensity in corresponding tissues were specific to age or developmental stage. The analysis of isozyme polymorphism in Leucaena tissues revealed at least 6 bands, which appeared to represent 4 loci. The comparison of the peroxidase frequencies in seven taxa, including three groups of the L. diversifolia complex, indicated that accessions of diploid L. diversifolia were highly variable in peroxidase isozyme pattern, in agreement with their morphological variation. However peroxidase isozyme patterns in three tetraploid taxa, L. pallida, L. diversifolia and L. leucocephala were found to be invariable.
The available morphological, cytological and distributional evidence suggests that Leucaena pallida is the amphiploid derivative of diploid L. diversifolia and L. esculenta. Based on morphological and distributional data, an accession from Guatemala, K740, and L. greggii are also considered to be amphiploids.
The studies indicate that the Leucaena diversifolia complex includes two species, L. pallida and L. diversifolia. The two cytotypes of the latter species are treated subspecies, L. diversifolia (Schletch.) Benth. ssp. diyersifolia (tetraploids), and L. diversifolia (Schletch.J ssp. trichandra Pan et Brewbaker, stat. nov. (diploids).
The evolution of the observed pattern of differentiation and divergence in the genus Leucaena is also discussed.
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Ph.D. - Agronomy and Soil Science|
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