Academic journal article Genetics

A Parthenogenesis Gene Candidate and Evidence for Segmental Allopolyploidy in Apomictic Brachiaria Decumbens

Academic journal article Genetics

A Parthenogenesis Gene Candidate and Evidence for Segmental Allopolyploidy in Apomictic Brachiaria Decumbens

Article excerpt

APOMIXIS, asexual reproduction through seed, results in progeny that are genetically identical to the female parent (Asker and Jerling 1992). Apomictic reproduction is found naturally in many economically important forage grass genera and is highly desirable in the sense that superior heterozygous genotypes can be propagated faithfully through seed over many generations without the expense and difficulty of hybrid seed production from inbred parental lines or vegetative propagation. Apospory, a common form of apomixis found in Paniceae grass genera including Brachiaria (Trin.) Griseb. (syn. Urochloa), Cenchrus L./Pennisetum Rich., Panicum L. (syn. Megathrysus), and Paspalum L. involves two sequential processes. An unreduced embryo sac first develops from an adjacent somatic nucellar cell (apomeiosis), which then develops into a viable embryo without fertilization (parthenogenesis) (Barcaccia and Albertini 2013; Hand and Koltunow 2014). Recently, the ASGR-BBML gene family has been postulated as candidate genes for parthenogenesis in the genus Cenchrus/Pennisetum (Conner et al. 2015). However, the role of the ASGR-BBML has yet to be established in other apomictic species and many questions remain regarding the role of apomixis in genomic stability and polyploid evolution. To date, these questions have largely been addressed through cytogenetic studies (e.g., Mendes-Bonato et al. 2002) or population genetics (e.g., Akiyama et al. 2011; Lovell et al. 2013). The development of dense genetic maps in apomictic species would complement these studies and facilitate molecular breeding in tropical forages, but factors including lack of investment, self-incompatibility, multisomic inheritance, and high levels of heterozygosity have thus far prevented the construction of saturated linkage maps in polyploid apomicts. Brachiaria forage grasses are widely sown across the global, and especially American tropics, where they have drastically increased the efficiency of cattle production, particularly in areas with marginal soils (Miles et al. 2004).

Brachiaria grasses are most economically important in Brazil, where they are planted on 99 Mha, accounting for 85% of sown pasture land (Jank et al. 2014). In addition to extensive pasture systems in Latin America, Brachiaria is also planted in intensive smallholder systems in Africa and Asia (Hare et al. 2013; Khan et al. 2014; Maass et al. 2015). The three most important commercial species, Brachiaria brizantha (A. Rich.) Stapf (palisadegrass), B. decumbens Stapf (signalgrass), and B. humidicola (Rendle) Schweick (koroniviagrass) exist primarily as apomicts with varying levels of polyploidy, although diploid sexual genotypes are also found in nature (Valle and Savidan 1996). B. brizantha and B. decumbens form an agamic complex with a diploid sexual species of lesser commercial importance, B. ruziziensis Germain & Evrard (ruzigrass) (2n=2x = 18) (Lutts et al. 1991). Microsporogenesis occurs normally in apomictic plants; therefore, apomictic genotypes can be crossed to sexual plants as pollen donors to generate progeny segregating for reproductive mode. Thus, the development of a synthetic autotetraploid sexual (2n = 4x =36) B. ruziziensis genotype through colchicine doubling (Swenne et al. 1981) facilitated recombination between sexual plants and tetraploid apomictic Brachiaria pollen donors and enabled the establishment of breeding programs at the International Center for Tropical Agriculture (CIAT) and Empresa Brasileira de Pesquisa Agropecuária (EMBRAPA) (Brazilian Enterprise for Agricultural Research) in the late 1980s (Miles 2007).

Efficient and reliable discrimination between apomictic and sexual offspring is of paramount importance to Brachiaria breeding programs because only apomict genotypes can be released as true-breeding cultivars (Miles 2007). Apospory is usually inherited as a single dominant Mendelian factor denoted as the apospory-specific genomic region (ASGR) (Ozias-Akins and van Dijk 2007). …

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