Citogenética de híbridos interespecíficos interploides e aneuploide do complexo brizantha (Urochloa P. Beauv.)

Abstract

Some species of the genus Urochloa are widely known for their foraging potential. Breeding programs are always aimed at developing new cultivars and one of the main strategies is interspecific crosses. In the brizantha complex, interspecific crosses mainly involve artificial tetraploid of U. ruziziensis, as female parents, and apomictic ones of U. brizantha or U. decumbens, as male parents. Often these crosses are carried out in blocks of polycrosses and without previous emasculation of the plants, so that the certification of hybrids and identification of pollen donors can be difficult. In addition, evaluating the chromosomal complement and meiosis of hybrids is important, as it can influence not only their development, but also the success of subsequent generations. Therefore, this study aimed to evaluate hybrids of the brizantha complex, from three approaches: i) identify the pollen donors of each hybrid; ii) characterize the somatic chromosomal complement; and iii) to evaluate meiosis and pollen viability, focusing on chromosome pairing. In the first study, the male parents were identified using microsatellite markers and two hybrids were identified as interploid (2n = 40 and 41 chromosomes) and one aneuploid (2n = 36+1). Genome size and number of ribosomal DNA sites were also determined for all genotypes, with variations found for both traits between parents and hybrids. The second study identified univalent and multivalent configurations in the diakinesis of the three hybrids, although the bivalent pairing was more frequent. Genomic in situ hybridization (GISH) in the interploid hybrid 7F/153 (U. ruziziensis x U. brizantha 5x) identified intragenomic and intergenomic pairings involving the B/B1 and B2 genomes. The predominant meiotic abnormalities involved errors in chromosome orientation and segregation, which can lead to the formation of aneuploid gametes. The GISH also contributed to identify a set of nine B2

chromosomes in the complement of Urochloa brizantha 5x (2n = 45), revealing its origin from unreduced gamete of a tetraploid and reduced gamete of diploid U. ruziziensis (B2B 2 ), and validating the transmission of four B2

chromosomes and eighteen B/B1

chromosomes to the hybrid. The results will contribute to breeding programs of the genus and offer new perspectives on chromosomal constitution and inheritance in interploid crosses or with the same ploidy level.