A Review of Unreduced Gametes and Neopolyploids in Alfalfa: How to Fill the Gap between Well-Established Meiotic Mutants and Next-Generation Genomic Resources

Abstract

The gene flow mediated by unreduced gametes between diploid and tetraploid plants of the Medicago sativa–coerulea–falcata complex is pivotal for alfalfa breeding. Sexually tetraploidized hybrids could represent the best way to exploit progressive heterosis simultaneously derived from gene diversity, heterozygosity, and polyploidy. Moreover, unreduced gametes combined with parthenogenesis (i.e., apomixis) would enable the cloning of plants through seeds, providing a unique opportunity for the selection of superior genotypes with permanently fixed heterosis. This reproductive strategy has never been detected in the genus Medicago, but features of apomixis, such as restitutional apomeiosis and haploid parthenogenesis, have been reported. By means of an original case study, we demonstrated that sexually tetraploidized plants maintain apomeiosis, but this trait is developmentally independent from parthenogenesis. Alfalfa meiotic mutants producing unreduced egg cells revealed a null or very low capacity for parthenogenesis. The overall achievements reached so far are reviewed and discussed along with the efforts and strategies made for exploiting reproductive mutants that express apomictic elements in alfalfa breeding programs. Although several studies have investigated the cytological mechanisms responsible for 2n gamete formation and the inheritance of this trait, only a very small number of molecular markers and candidate genes putatively linked to unreduced gamete formation have been identified. Furthermore, this scenario has remained almost unchanged over the last two decades. Here, we propose a reverse genetics approach, by exploiting the genomic and transcriptomic resources available in alfalfa. Through a comparison with 9 proteins belonging to Arabidopsis thaliana known for their involvement in 2n gamete production, we identified 47 orthologous genes and evaluated their expression in several tissues, paving the way for novel candidate gene characterization studies. An overall view on strategies suitable to fill the gap between well-established meiotic mutants and next-generation genomic resources is presented and discussed.

Asexual reproduction through seeds: the complex case of diplosporous apomixis

“…falcata), plants that produce unreduced female and male gametes were used for sexual polyploidization and breeding (Veronesi et al, 1986). The locus controlling 2n egg cell formation in two-n egg (tne), a spontaneous alfalfa mutant, has been identified; however, the TNE gene has not been cloned yet (Palumbo et al, 2021). Rhoades and Dempsey (1966) described a maize natural recessive mutant named elongate (el) because of the elongated appearance of the chromosomes at both meiotic anaphases, which produced unreduced embryo sacs, but reduced functional pollen grains.…”

Exploiting Unreduced Gametes for Improving Ornamental Plants

“…SMC5/6 mutants show an absence of chromosome segregation during the first and/or second meiotic division, producing 2 n gametes. A comparison of the well-established meiotic mutants in alfalfa with the genes identified from Arabidopsis showed that nine proteins belonging to A. thaliana known for their involvement in 2 n gamete production occurred in alfalfa, suggesting common lineage of genes implicated in 2 n gamete formation ( Palumbo et al, 2021 ). Molecular techniques, particular CRISPR/Cas9 could be used for engineering plants with increased production of either 2 n pollen or 2 n eggs and used for breeding of novel polyploid ornamental plants.…”

Natural variation in potato sexual reproduction facilitates breeding

“…However, the production of unreduced gametes with a somatic (2n) rather than a gametophytic (n) number of chromosomes has been observed in a wide variety of plant species (Harlan and de Wet, 1975;Kreiner et al, 2017a;Ramanna and Jacobsen, 2003). When those unreduced gametes participate in fertilization, polyploid offspring are produced in a process known as sexual polyploidisation (Palumbo et al, 2021). Unreduced gametes can result from a variety of cytological events that include pre-or post-meiotic genome doubling and several meiotic restitution mechanisms (de Storme and Geelen, 2013).…”