Breeding Tetraploid Paspalum simplex: Hybridization, Early Identification of Apomicts, and Impact of Apomixis on Hybrid Performance

Abstract

Apomictic grasses predominate among tropical forages, and specific breeding techniques are needed for their genetic improvement. The objectives of this study were (i) to generate hybrids by crossing tetraploid sexual and apomictic Paspalum simplex Morong genotypes, (ii) to develop a technique based on molecular markers for early identification of apomictic hybrids, and (iii) to determine the relationship between mode of reproduction and performance of hybrids. Crosses were made between two induced sexual and seven apomictic tetraploid plants. Identification of apomictic hybrids during seedling stage was performed using a marker linked to apomixis, and by flow cytometric seed analysis. Hybrids were evaluated under field conditions for plant diameter and height, initial growth, and seasonal regrowth during two consecutive growing seasons. Sexual tetraploid plants used as female parents behaved as allogamous, since 95% of the progeny had male‐specific markers. The ratio between apomictic and sexual hybrids differed from 1:8.7 to 1:0.6 among crosses, with a mean of 1:2.4. There was a 96% coincidence between the use of the apomixis‐linked marker and flow cytometric seed analysis. A technique based on the use of early DNA isolation and the amplification of a molecular marker linked to apomixis was developed. We found no overall difference between apomictic and sexual hybrids for the evaluated traits, except for fall regrowth in the first year, for which apomictic hybrids were superior. Generation of large tetraploid hybrid progenies is possible in P. simplex. Segregation for mode of reproduction depends on parents involved. Agronomic performance is not related to reproductive mode among tetraploid hybrids.

How to Become an Apomixis Model: The Multifaceted Case of Paspalum

“…Since then, large segregating progenies have been generated by crossing these artificially induced sexual tetraploid genotypes as female parents and apomictic ecotypes as pollen donors. This procedure has been repeatedly used over the last years attempting to improve P. notatum [ 72 , 73 , 74 , 75 ] and, more recently, P. simplex [ 131 ] and species of the Plicatula group [ 70 ]. The experience with P. notatum indicates that only a reduced fraction of the progeny (around 10%) inherits the full capacity to express apomixis [ 72 , 73 , 74 ] and variable degrees of apospory expressivity are observed within hybrids [ 73 , 74 , 75 ].…”

“…Moreover, the self-incompatibility present in the diploid germplasm [ 132 ] and the induced sexual tetraploids is not transmitted to the sexual or apomictic progeny [ 72 ]. There is also evidence indicating that segregation for apomixis is independent of the segregation for traits of agronomic interest in P. simplex [ 131 ]. In the Plicatula group, several intra and interspecific crosses have been performed and different degrees of crossability and fertility of the resulting hybrids were detected [ 70 ].…”

“…MMs have been used to monitor the process of hybridization between sexual and apomictic genotypes in Paspalum , particularly for the identification of true hybrids within progeny [ 35 , 70 , 74 , 131 ]. The general idea is to test the progeny for the presence of male-specific MMs and for the segregation of female-specific markers indicative of recombination events in the female side.…”

“…Moreover, several markers 100% linked to apospory have also been developed for the genus [ 77 , 85 , 137 ]. Some of these markers have been successfully applied to identify hybrids exhibiting apospory in P. notatum [ 74 , 75 ] and P. simplex [ 131 ]. Usually, they are useful for an initial evaluation that allows separating all the highly sexual progeny from the aposporous, but further analyses are needed to determine the different levels of apospory expressivity.…”

Reproductive Systems in Paspalum: Relevance for Germplasm Collection and Conservation, Breeding Techniques, and Adoption of Released Cultivars

“…These kinds of studies allowed the development of molecular markers linked to apospory in P. notatum (Pupilli et al, 2001; Martínez et al, 2003; Pupilli et al, 2004; Stein et al, 2004; Stein et al, 2007, Rebozzio et al, 2012) and P. simplex (Labombarda et al, 2002). The availability of molecular markers allows breeders to achieve early classification of reproductive mode in segregating progenies, saving time and resources (Zilli et al, 2015; Brugnoli et al, 2019). Due to variable apospory expressivity reported in Paspalum species, this technique does not allow identification of highly apomictic hybrids, therefore auxiliary techniques such as mature embryo sac observation, flow cytometry, or progeny tests, using morphological or molecular markers, are needed.…”