Chaotic spindles causes abnormal chromosome behavior in an interspecific sexual hybrid of Urochloa

Intra- and interspecific hybridization in the genus Urochloa is complex due to apomixis and polyploidy. Such barriers can be overcome by crossing apomictic tetraploid plants as pollen donors with artificial tetraploid sexual plants as female parents. Another barrier is the incompatibility between the genomes of the different species used in hybridization. Since the meiotic behavior is a good indication of affinity between genomes in a hybrid, the present study analyzed the meiotic behavior of a sexual polyploid interspecific hybrid of Urochloa from the breeding program at Embrapa Beef Cattle. In addition to the segregational abnormalities associated with polyploidy, cytological studies revealed that, after diakinesis, chromosomes presented an abnormal arrangement in the metaphase plate. In 31.33% of meiocytes, chromosomes were organized into more than one plate, forming up to four metaphase plates. Another abnormality observed in 5.72% of the cells was the lack of convergence of chromosomes to the equatorial plate and consequent abnormal anaphase segregation. These abnormalities led to the formation of several nuclei within the same cell then separated by an irregular cytokinesis pattern, forming tetrads with micronuclei, tetrads with microcytes and polyads. Although the sexual hybrid S44 was selected due to its good agronomic characteristics, such as resistance to pasture spittlebugs, the main insect pest in Urochloa , the abnormal meiotic behavior seriously may compromise the formation of viable gametes and consequent formation of viable seeds.