Marisa Toniolo Pozzobon

Evidence of sexuality in induced tetraploids of Brachiaria brizantha (Poaceae)

Difficulties in obtaining new breeding lines of Brachiaria (Trin.) Griseb., an important forage grass in Brazil, are mostly related to differences in ploidy among the accessions, and to apomixis, an asexual mode of reproduction. Usually, sexual accessions are diploid while apomicts are polyploid. Induced tetraploids of Brachiaria brizantha (A. Rich.) Stapf have been successfully obtained and this paper presents the results of a study of their reproductive modes and fertility. Despite frequent meiotic aberrations during microspore development, the induced tetraploids produced viable pollen and produced progeny after controlled self-pollination. Similarly to the original diploid sexual progenitor, embryo sacs of the Polygonum type with confirmed meiotic origin were present in the induced tetraploids suggesting chromosome doubling did not alter the reproductive mode. The embryo sac of the Polygonum type was also observed in progenies obtained after self and open pollination. Nevertheless, embryo sacs of the Polygonum and the Panicum types within the same ovule were observed in some progenies obtained after open pollination, probably having resulted from hybridization with tetraploid, apomictic plants. Indeed, the compatibility of the progeny with tetraploid, apomictic B. brizantha was confirmed by the formation of mature caryopses after controlled pollination. Evidence is presented that the induced tetraploids and their progeny are sexual plants and that they are compatible with natural tetraploid B. brizantha. The induced tetraploids will be useful for analyses of apomictic inheritance as well as in the development of sexual tetraploid lines in Brachiaria breeding programs.

Chromosome counts in brazilian species of Paspalum L. (Gramineae)

Germplasm accessions obtained in collecting missions covering all the five regions of Brazil were submitted to cytological analysis, either in mitosis or meiosis. Chromosome numbers are reported from 107 accessions of 33 species, representative of 13 of the informal taxonomic groups, in which the genus is traditionally divided. In the Plicatula group, under intensive taxonomic investigation, 27 accessions are mentioned without any specific identification. The Southern region is very well represented, with 65 accessions, followed by the West Central region, with 33. The chromosome numbers of P. fillfolium (2n = 20), P hartwegianum (2n = 60), P. rhodopedum (2n = 40) and P. wettsteinii (2n - 40 and 60) are reported for the first time. Also new are the reports of 2n = 50 for P. plenum and for a yellow-anthered form of P dilatatum. The new findings include an additional diploid accession of P plicatulum (2n = 20) and a second accession of P. mandiocanum with 2n = 50, a number in disagreement with the most frequent hexaploid level in this species. Additional data confirm previous counts, thus extending their geographic amplitude.

Cytogeography and variation of stomatal size of Paspalum Glaucescens (Gramineae; Paniceae) in Southern Brazil

Paspalum glaucescens belongs to the informal group Plicatula, reproductively characterized by the dominance of tetraploid apomitic lines in most of its populations, with rare diploid, sexual counterparts. The species shows high phenotypic variation. Twenty nine Southern Brazilian accessions were cytologically and morphologically analysed. Most of the accessions were tetraploid (2n = 4x = 40). Meiotic study of three tetraploids showed their irregular behaviour. Eight accessions presented the diploid level (2n = 2x = 20).This can be considered a very high frequency of diploids in a member of the Plicatula group. While the tetraploids are concentrated at the highest elevations, diploids were detected mostly in the lowlands. Average stomatal sizes were quite distinct on different ploidy levels, being larger in tetraploids.

Cytogenetic analyses in Paspalum L. reveal new diploid species and accessions

Chromosome numbers were counted in 126 new accessions of 50 Paspalum species from Brazil, Argentina, Paraguay and Bolivia. The chromosome numbers 2n=12, 20, 24, 30, 40, 50, 60, 80 were confirmed. Chromosome numbers for P. arenarium (2n=20), P. barretoi (2n=20), P. aff. ceresia (2n=40), P. corcovadense (2n=20), P. crispulum (2n=20), P. flaccidum (2n=40), P. nummularium (2n=20), P. scalare (2n=20), P. vescum (2n=20) and P. rectum (2n=20) and a diploid cytotype of P. malacophyllum are reported for the first time. The predominance of tetraploid accessions (43.6%) was confirmed, but an unusually high number of diploid species (44%) and accessions (35.7%) was found. These results open new perspectives for breeding programs, phylogenetic studies, and for research on apomixis control, since diploids of Paspalum are typically sexual.

Chromosome number in Brazilian germplasm accessions of Paspalum hydrophilum, P. modestum and P. palustre (Gramineae; Paniceae)

This paper compiles results of chromosome counts of Paspalum hydrophilum, P. modestum and P. palustre. Four Brazilian accessions of P. modestum have shown 2n = 2x = 20 chromosomes, a number already found in one accession from Argentina and in two from Brazil. Three other Brazilian accessions showed tetraploid level (2n = 4x = 40), which was previously unknown in this species. In P. hydrophilum, only one of the accessions analyzed presented tetraploid level, initially established for the species from plants collected in Argentina. Five additional accessions from Brazil showed the diploid number, previously detected in a single Brazilian population. A tetraploid cytotype was found in P. palustre, previously known as a diploid species. In addition to confirming the occurrence of distinct ploidy levels for all three species, the results establish the predominance of the diploid level in P. hydrophilum and P. modestum accessions collected in Brazil.

Cytological and reproductive aspects in the Caespitosa group of Paspalum.

Somatic chromosome numbers are reported for thirty four germplasm accessions of Paspalum, Caespitosa group, representing five different species. All five species have shown x=10 as the basic chromosome number. The diploid 2n=20 chromosome number was confirmed for P. chacoense and P. indecorum, as well as sexuality for the latter. This is the first report of the chromosome number and cytological behavior for P. ligulare (2n=20 and 40), P. pleostachyum (2n=20, 30 and 40) and P. redondense (2n=20 and 40). The present results document regular meiosis in diploid accessions, with primarily bivalent pairing at diakinesis and metaphase I. Polyploids of these species had irregular meiosis, with univalent, trivalent, and quadrivalent chromosome associations. Diploid accessions of P. pleostachyum and P. ligulare have shown a single meiotic embryo-sac, indicating sexual reproduction, while the triploid and one of the tetraploid accessions of P. pleostachyum have shown aposporic embryo-sacs of nucelar origin, and a few meiotic sacs, suggesting facultative apomixis. Identification of the new diploid accessions may prove useful for phylogenetic studies of Paspalum, as well as for breeding programs focusing on the forage potential of species of the Caespitosa group.