Bahiagrass is a perennial warm-season grass and one of the major native grasslands in the New World. It is also currently one of the most important pasture and utility turf species of Florida and the southern Coastal Plain region of the United States. This diverse species contains germplasm with different ploidy levels and linked reproductive characteristics. Diploids reproduce sexually, while tetraploids reproduce asexually by apomixis. Both cytotypes have been introduced into the southern United States and extensively cultivated for more than 60 years.
Apomixis is an asexual mode of reproduction where megasporogenesis and fertilization of the egg are bypassed to result in the production of clonal progeny. This trait, therefore, allows perpetuation of fixed genotypes. The ability to fix highly complex genotypes, including high-yielding hybrids, through apomixis can have a tremendous advantage in plant breeding and seed production.
Apomixis has been reported in more than 125 grass species. A majority of these are C4 grasses, especially members of the Panicoideae and Chloridoideae subfamilies. The tetraploid germplasm of bahiagrass offers an excellent opportunity for manipulating apomixis to generate highly productive hybrids that may result in new forage or turf cultivars.
Apomixis in bahiagrass includes the formation of unreduced embryo sacs from nucellar cells (apospory), the pathenogenetic development of embryos, and the development of the endosperm following fertilization of the polar nuclei (pseudogamy).
Apospory in this species is inherited as a single dominant Mendelian factor with distorted segregation. This factor is located in a genomic region characterized by suppression of recombination and preferential chromosome pairing. Synteny of molecular markers was detected between the apospory controlling region and specific regions of rice (Oryza sativa L.) chromosomes 2 and 12.
All bahiagrass tetraploid cultivars released in the United States were superior apomictic ecotypes selected from introduced germplasm. The cultivars Argentine (PI 148996), Paraguay 22 (PI 158822), and Wilmington (PI 434189) were popularized from early evaluation studies for forage yield and adaptation in the southern United States.
The tetraploid germplasm of bahiagrass, Paspalum notatum Flüggé, is an unexploited source of variability that can be used for the genetic improvement of this species as forage or turf.
A group of researchers from Florida and Georgia have studied bahiagrass to develop a segregating population by hybridizing induced sexual and apomictic clones and to characterize the resulting population for mode of reproduction, seed fertility, growth habit, seasonal growth pattern, and frost resistance.
In this research study, a population of 591 hybrids had a 4:1 ratio between sexual and apomictic (facultative + obligate apomictic) plants and an 8:1 ratio between others (facultative apomictic + sexual) and obligate apomictic plants. The general fertility of the progeny was higher than that of the induced sexual parents. Significant differences were observed for self- and cross-fertility among apomictic progeny and for self-fertility among sexual progeny.
Most sexual progeny set the same amount of seed when self- or cross- pollinated, indicating an absence of self-incompatibility. The researchers observed marked variation for growth habit, seasonal growth patterns, and frost resistance among the progeny. Heritability estimates obtained for these characteristics indicated that selection can be successfully used for the genetic improvement of this species.
Feature developed from:
Bahiagrass Tetraploid Germplasm: Reproductive and Agronomic Characterization of Segregating Progeny by C. A. Acuña, A. R. Blount, K. H. Quesenberry, K. E. Kenworthy and W. W. Hanna, Published online 17 March 2009 in Crop Sci 49:581-588 (2009).