Cytogenetic studies of Lolium multiflorum Lam., Festuca arundinacea Schreb., their hybrids and amphidiploids

Abstract

Plant breeders intercross Lolium multiflorum and Festuca arundinacea with the purpose of obtaining hybrids which combine agronomically interesting characters of the parent species. The end result can be an amphidiploid, or the transfer of a limited number of genes from one species to the other. Especially in the first case, meiotic regularity often is a bottle neck. In the present study the influence of various Festuca and Lolium parents on chromosome pairing, further meiotic behaviour and fertility in the hybrids and amphidiploids was studied.In F. arundinacea meiosis is somewhat irregular due to the presence of univalents and multivalents and lack of stability. For the same plant significant differences in number of chromosome. arms bound by chiasmata were found over two years, and a number of plants even showed significant differences within clones within years. Significant differences in meiotic chromosome association were found between the Festuca varieties. Between plants of the same variety, differences were also found, but were smaller than between varieties. Chromosome association seems to have little effect on pollen stainability and fertility.Diploid Lolium had very regular meiosis and most of the chromosomes formed ring-bivalents at first metaphase. Tetraploid Lolium showed some uni- and trivalents, and quadrivalents were found with very low frequency (average 1.24 per cell), for an autotetraploid.Hybrids were obtained easily between diploid L. multiflorum varieties Lior and Manawa (as female parent) and F. arundinacea . The percentage of seed set was high: 44% for the cross Lior x Festuca and 50% for Manawa x Festuca. Some differences in seed set between the Festuca parents were found with Ludion as the variety with the highest seed set. Diploid Lolium varieties did not influence seed set but influenced germination. The crosses with the variety Manawa had a significantly higher percentage of germination (76%) than the crosses with Lior (50%). The reciprocal cross gave a very low percentage seed set (0.75%), and only one hybrid seed germinated. The seed set of the crosses with the tetraploid Lolium as female parent was much lower (19%) than that of the diploid Lolium and the germination of the hybrid seeds was very low.The hybrids (2n = 28) showed a high level of meiotic chromosome association and averages of 10.31 bivalents for the hybrids Lior x Festuca and 10.68 bivalents for Manawa x Festuca were found. Univalents, tri- and quadrivalents were observed frequently. The differences between the hybrids in numbers of univalents and numbers of chromosome arms bound by chiasmata were highly significant, but no specific influence of the Festuca nor the Lolium parent could be detected. Pollen stainability was very low and the hybrids were completely male sterile. Pollen development was almost normal until first pollen mitosis, after which the pollen grains degenerated. This might be due to a genetic unbalance and it is possible that pollen grains completing first pollen mitosis had all seven Lolium chromosomes present.In the amphidiploids most of the chromosomes paired as bivalents (average 22.29 for the amphidiploids Lior x Festuca and 24.31 for Manawa x Festuca). In comparison to the chromosome pairing in the hybrids a higher number of multivalents could be expected. This was not the case and pairing was mostly limited to bivalents. All amphidiploids showed univalents and multivalents. As for the hybrids, significant differences in numbers of univalents and of arms bound were found between the amphi-diploids. As observed for the hybrids, the chromosome pairing in the amphidiploids depended on the combination of the parents rather than on the specific genotype of the varieties used. Chromosome pairing in the amphidiploids was not influenced by temperature in the range between 15 and 25 °C. C 1 plants with 56 chromosomes did not show a more regular meiosis than C 0 plants.Comparison between the tetraploid and pentaploid (from tetraploid Lolium x Festuca) hybrids with their corresponding amphidiploids showed that although most amphidiploids had a higher number of arms bound per chromosome than their hybrid, only for four octoploid amphidiploids the differences were significant. Chromosome pairing thus increased only slightly or not at all (decaploids) after chromosome doubling.The number of micronuclei per tetrad (M/T) proved to be a highly variable character and was not always correlated with the number of univalents.However, this character can still be useful for selection. Plants with a very low number of M/T perhaps have a low number of univalents but most probably a relatively good pollen stainability, gemination of the pollen grains and fertility.Comparison between the Festuca parents, the hybrids and the amphidiploids in respect to the number of arms bound per chromosome (table 23) showed no significant differences between the parents and the hybrids and between the parents and the amphidiploids. For the same number of arms bound per chromosome, Festuca had a more regular meiosis with only homologous pairing and the amphidiploids an irregular meiosis with mainly homologous but probably also homoeologous pairing, or no pairing at all. It is likely that chromosome pairing in F. arundinacea , which is restricted to bivalents, is under genetic control. It seems that the Lolium genome partially suppresses the action of the pairing regulating genes of F. arundinacea . This could be the cause of the meiotic instability of the amphidiploids.Calculation of the relative affinity showed that in the hybrids two genomes are more closely related to each other than they are to a similarly related second pair of genomes, The Lolium genome is closely related to the genome of F. pratensis , which probably is the donor of one of the genomes of F. arundinacea . The two other genomes of F. arundinacea are closely related so that in haploid state, their chromosomes can pair.In this study, even with very distinct Festuca parents and significant differences in chromosome pairing, no amphidiploid with a stable meiosis was found. The fact that between the amphidiploids significant differences in numbers of arms bound were observed may indicate that certain genetic combinations give a regular meiosis but the chance of finding such a genotype is very small. If the amphidiploids cannot be used directly for breeding purposes, introgression of some characters from one into the other parental species seems to be the most promising way of using the amphidiploids between L. multiflorum and F. a rundinacea . Since homoeologous pairing occurs, introgression is possible