The selection of productive varieties of modern Criollo cocoa, showing fine aromatic qualities in their beans, is of major interest for some producing countries, such as Venezuela. Cultivated populations of Modern Criollo or Trinitario varieties may be suitable for admixture mapping analysis, as large blocks of alleles derived from two identified divergent ancestors, recently admixed, are still preserved, after a few generations of recombination, similar to experimental mapping progenies. Two hundred and fifty-seven individuals from a cultivated population of Modern Criollo were selected and analysed with 92 microsatellite markers distributed along the genome. This population exhibited a wide range of variability for yield factors and morphological features. Population structure analysis identified two main subgroups corresponding to the admixture from the two ancestors Criollo and Forastero. Several significant associations between markers and phenotypic data (yield factors and morphological traits) were identified by a least squares general linear model (GLM) taking into account the population structure and the percentage of admixture of each individual. Results were compared with classical QTL analyses previously reported for other cacao populations. Most markers associated to quantitative traits were very close to QTLs detected formerly for the same traits. Associations were also identified between markers and several qualitative traits including the red pigmentation observed in different organs, mainly associated to common markers in linkage group 4.
A sound understanding of crop history can provide the basis for deriving novel genetic information through admixture mapping. We confirmed this, by using characterization data from an international collection of cocoa, collected 25 years ago, and from a contemporary plantation. We focus on the trees derived from three centuries of admixture between Meso-American Criollo and South American Forastero genomes. In both cacao sets of individuals, linkage disequilibrium extended over long genetic distances along chromosome regions, as expected in populations derived from recent admixture. Based on loose genome scans, genomic regions involved in useful traits were identified. Fifteen genomic regions involved in seed and fruit weight variation were highlighted. They correspond to ten previously identified QTLs and five novel ones. Admixture mapping can help to add value to genetic resources and thus, help to encourage investment in their conservation.
Cytological and biochemical studies were conducted in the genus Leucaena in relation to the breeding of low mimosine, high yielding forage types of L. leucocephala (Lam.) de Wit. Meiotic and mitotic chromosomes were studied in the following species; L. leucocephala and L. pulverulenta (Schlecht) Benth. (2n = 56); L. trichodes Benth., L. lanceolata (S. Wats.), L. stenocarpa (Urban), and accessions designated “L. buitenzorg” (2n = 52) and “L. esculenta” (2n = 104).All species of Leucaena hybridized freely, insofar as they were tested. The 80‐chromosome hybrids of L. pulverulenta ✕ L. leucocephala were grown and studied in detail. Seed recovery and yields of these hybrids were high, despite meiotic chromosome pairing of 26 bivalents and 28 univalents. F2 plants ranged widely in chromosome numbers.Assays for the toxic alkaloid, mimosine, were conducted on F1, F2, and backcross progenies from a cross of high ✕ low mimosine strains of L. leucocephala. Despite the apparent polyploidy of the species, wide variations were observed in mimosine contents of the segregating generations, enabling the selection of plants with less than 30% of the normal mimosine values (ca. 4% dry weight) of tropical forage strains. The correlation between mimosine and protein contents for related F2 segregants from high ✕ low mimosine crosses was not significant (r = .60).
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