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Evidence of allopolyploidy in Urochloa humidicola based on cytological analysis and genetic linkage mapping


The African species Urochloa humidicola (Rendle) Morrone & Zuloaga (syn. Brachiariahumidicola (Rendle) Schweick.) is an important perennial forage grass foundthroughout the tropics. This species is polyploid, ranging from tetra to nonaploid, andapomictic, which makes genetic studies challenging; therefore, the number of currentlyavailable genetic resources is limited. The genomic architecture and evolution of U.humidicola and the molecular markers linked to apomixis were investigated in a full-sibF1 population obtained by crossing the sexual accession H031 and the apomicticcultivar U. humidicola cv. BRS Tupi, both of which are hexaploid. A simple sequencerepeat (SSR)-based linkage map was constructed for the species from 102polymorphic and specific SSR markers based on simplex and double-simplex markers.The map consisted of 49 linkage groups (LGs) and had a total length of 1702.82 cM,with 89 microsatellite loci and an average map density of 10.6 cM. Eight homologygroups (HGs) were formed, comprising 22 LGs, and the other LGs remainedungrouped. The locus that controls apospory (apo-locus) was mapped in LG02 andwas located 19.4 cM from the locus Bh027.c.D2. In the cytological analyses of somehybrids, bi- to hexavalents at diakinesis were observed, as well as two nucleoli in somemeiocytes, smaller chromosomes with preferential allocation within the first metaphaseplate and asynchronous chromosome migration to the poles during anaphase. Thelinkage map and the meiocyte analyses confirm previous reports of hybridization andsuggest an allopolyploid origin of the hexaploid U. humidicola. This is the first linkagemap of an Urochloa species, and it will be useful for future quantitative trait locus (QTL)analysis after saturation of the map and for genome assembly and evolutionary studiesin Urochloa spp. Moreover, the results of the apomixis mapping are consistent withprevious reports and confirm the need for additional studies to search for a cosegregatingmarker. (AU)

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