Caracterização citogenética de acessos de Paspalum (Poaceae) com potencial para seleção direta de cultivares

Abstract

The genus Paspalum comprises about 400 species distributed in the tropical and subtropical regions of the Americas. A number of species furnishes native pastures of quality in the South America. In the genus, most of species are polyploid and polyploidy is related to apomixis that compromises the breeding purposes because apomictic plants could be used only as pollen donors in crosses. In the last years, several germplasm collection trips were organized by Embrapa Genetic Resources and Biotechnology aiming to establish an wide basic collection for the genus. Part of this collection is stored at Embrapa Cattle-Southeast (CPPSE) located in São Carlos, SP, Brazil. This germplasm is used as source of genetic variability for breeding programs to obtain cultivars for different purposes. After several years of selection for agronomic traits, 20 accessions of different species were selected for new studies to create new cultivars. Among them, cytogenetic studies were included. Eighteen accessions of 10 species were analyzed by conventional methodology for the determination of their chromosome numbers and ploidy level, and the characterization of their meiotic behavior. It was found that 14 accessions were tetraploid (2n = 4x = 40); two pentaploids (2n = 5x = 50) and two hexaploids (2n = 6x = 60). Among the analyzed species, two (P. conspersum e P. mandiocanum) had the chromosome number reported for the first time. Tetraploidy and pentaploidy were described in P. conspersum. For the other species, the chromosome numbers recorded agree with the previous reports. Among the 14 tetraploid accessions, less then 30% of tetrads were abnormal, and in the majority of accessions the meiotic behavior was quite regular. In the hexaploid accessions, the meiosis was regular with chromosomes pairing as bivalents. In the two pentaplod accessions of P. conspersum and Paspalum sp., the chromosomes showed complete asynapsis and after a single meiotic division, unreduced gametes were formed. Although apomictics, polyploid plants demand viable gametes to fertilize the secondary nucleus of the embryo sac to ensure the endosperm formation and the formation of viable seeds. Thus, high seed production is related to meiotic stability. The tetraploid and hexaploid accessions analyzed are candidates to create new cultivars. Although pentaploid accessions of P. conspersum and Paspalum sp are source of 2n gametes, they should not be used until more studies elucidate the origin of assynapsis.