Abstract
The meiotic behavior of four interspecific promising hybrids was evaluated by conventional cytological methods. The female genitors were two artificially tetraploidized sexual accessions of B. ruziziensis (R41 and R44, 2n = 4χ = 36), which were crossed to an agronomically superior natural tetraploid apomictic genotype of B. brizantha (B140 - BRA003395). Three of them (HBGC313, HBGC 315, and HBGC324) were sexual and one (HBGC325) apomictic. Analyses of some cells in diakinesis revealed multivalent chromosome configurations, suggesting that genetic recombination and introgression of some genes could be present. The four hybrids had different types of meiotic abnormalities at various frequencies. Abnormalities related to irregular chromosome segregation due to polyploidy were common among these hybrids, and characterized by precocious chromosome migration to the poles, laggard chromosomes, both generating micronuclei in telophases and tetrads and, as a consequence, unbalanced gametes. One abnormality genotype-specific, related to spindle orientation (a putative divergent spindle mutation), was recorded for the first time in two of the hybrids, HBGC313 and HBGC325. The sexual hybrid HBGC324 had the lower rate of abnormalities, and it could be used as a female genitor in future crosses in the breeding program. The abnormalities present in these hybrids may impact fertility and affect seed production. Based on the results, HBGC324 is the single hybrid recommended to the breeding program. Hybrids must produce a good amount of viable seeds, besides good overall dry matter production and nutritive value, in order to be widely utilized and adopted in production systems. Due to pseudogamy, the desirable superior apomictic hybrids need viable pollen grains to fertilize the secondary nucleus of the embryo sac and thus ensure normal and vigorous endosperm development and plenty of seed set.
Highlights
In the last decades, Brazil became the largest beef cattle producer and exporter of beef products
Brachiariagrass alone accounts for 85% of the cultivated pastures in Brazil, covering over 80 million hectares and sustaining the largest commercial herd in the world – about 205 million of heads
The main objective of the Brachiaria program in tropical America is to use the sexuality of the tetraploid ruzigrass to release the genetic diversity locked in the natural tetraploid apomictics such as B. brizantha, and B. decumbens, to produce novel apomictic hybrid cultivars (Valle and Savidan, 1996; Miles and Valle, 1996; Miles et al, 2004)
Summary
Brazil became the largest beef cattle producer and exporter of beef products. The main objective of the Brachiaria program in tropical America is to use the sexuality of the tetraploid ruzigrass to release the genetic diversity locked in the natural tetraploid apomictics such as B. brizantha (palisadegrass), and B. decumbens (signalgrass), to produce novel apomictic hybrid cultivars (Valle and Savidan, 1996; Miles and Valle, 1996; Miles et al, 2004). The crosses were preferentially done between sexual accessions of B. ruziziensis and natural apomictic B. brizantha cv.
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