Cytogenetics of native species of Festuca, crossover analyses and chromatid interference in Festulolium

Abstract

Ryegrasses (Lolium L.) are ideal grasses for use as animal fodder in temperate pasture farming, however, their use is restricted as they are susceptible to biotic and abiotic stresses. Festuca L., in turn, presents more resistant species, but with lower performance when it comes to the provision of forage. The Lolium-Festuca complex has fascinated cytogeneticists and breeders for over a hundred years for some reasons. Firstly, due to the unusual and easy nature with which intergeneric hybrids (Festulolium) are obtained and have homeologous recombination, feature also present in hybrids of Allium L.. Secondly, due to its economic importance, mainly because it combines the best qualities of both genus: the high production of Lolium forage and the resistance of Festuca. Cytogenetic analyzes carried out with genomic in situ hybridization (GISH) made it possible to visually discriminate parental genomes, chromosomes/introgressed segments and monitor the behavior over generations. This technique, usually combined with fluorescent in situ hybridization (FISH), allows the composition description of the hybrid genome, providing valuable information for the production of new cultivars, selection of material in the early stages of breeding programs and characterization of wild native species. Despite the countless cytogenetic studies already carried out, the complex is still a fertile field for pioneering analyzes, such as for native species of Brazil and in the clarification of genetic phenomena not yet fully elucidated, such as, for example, chromosomal/chromatid interference in crossing-over. Therefore, the present work is divided into two manuscripts, which the objectives were: to build and characterize the karyotypes of two species native to Brazil, Festuca ulochaeta Steud. and Festuca fimbriata Ness., using FISH with 35 and 5S rDNA probes; to evidence the existence of chromosomal/chromatid interference by means of GISH in Festulolium hybrids (L. multiflorum Lam. x F. pratensis Huds.). The results with the first study were that both native species had 42 chromosomes and a pair of 5S rDNA sites. For the 35S rDNA site, two pairs were identified in F. fimbriata and one pair in F. ulochaeta. The chromosomal characteristics indicates that these species have a symmetrical karyotype and possible allopolyploid origin. In the second manuscript, through GISH in anaphase I cells of interspecific hybrids (L. multiflorum × F. pratensis and Allium cepa × A. Roylei), the action of the interference phenomenon in crossing-over observed. In the analysis it was possible to quantify the involvement of two to four chromatids in double crossovers and, in those that involved the maximum number of chromatids, the frequency was 64%, when the expected would be only 25%. It was also possible to provide the physical measure of the interference that covered 30 to 40% of the relative length of the chromosomal arm and that the centromere acts as a barrier for the propagation of the interference.