Abstract
Current Musa breeding strategies are complex and time consuming involving the selection of tetraploids from 3x - 2x crosses. Secondary triploids are then obtained by crossing these tetraploids with diploids. Considering the very low hybrid seed set, routine embryo rescue procedures of hybrid seeds and the long growth cycle of banana, it takes approximately 10 - 12 years to produce an acceptable banana hybrid. The banana breeding process could benefit tremendously if triploid bananas could be obtained directly from 2x - 2x crosses through the process of unilateral sexual polyploidization. There are few reports on the mechanisms through which Musa species produce 2n pollen. This study investigated the type of meiotic irregularities that lead to 2n pollen formation in diploid, triploid and tetraploid Musa accessions using cytological analyses. The results showed that aberrations in cytokinesis and karyokinesis during microsporogenesis are possible mechanisms for 2n pollen formation in Musa. The meiotic aberrations described in this study have implications for Musa breeding. It appears that 2n pollen formation in Musa occurs via both first division restitution (FDR) and second division restitution (SDR). FDR is said to be more promising in transferring more heterozygosity from parents to offspring.
Highlights
Meiosis is considered as a highly conservative process that leads to a reduction in the chromosome number in the gametes, mutations in the genes controlling the process lead to abnormalities, some of which can produce 2n gametes in plants (Pagliarini, 2000)
The results showed that aberrations in cytokinesis and karyokinesis during microsporogenesis are possible mechanisms for 2n pollen formation in Musa
It appears that 2n pollen formation in Musa occurs via both first division restitution (FDR) and second division restitution (SDR)
Summary
Meiosis is considered as a highly conservative process that leads to a reduction in the chromosome number in the gametes, mutations in the genes controlling the process lead to abnormalities, some of which can produce 2n gametes in plants (Pagliarini, 2000). The formation of 2n or diplogametes is a common feature in many plant species including wild potato (Camadro et al, 2008), Hibiscus (Van Laere et al, 2009), Begonia (Dewitte et al 2010), Turnera sidoides ( Kovalsky and Neffa 2016), Avena ventricosa (Nicoloudakis et al, 2018), lemon (Xie et al, 2020) and Cymbidium (Zeng et al, 2020). The production of 2n gametes in plants is considered to be a dominant process in the origin of polyploid crop species (Harlan and de Wet, 1975) as well as the development of cultivars (Lim et al, 2001). Musa is a polyploid complex that comprises diploid species and triploid and tetraploid accessions that originated from interand intraspecific hybrids between M. acuminata and M. balbisiana. The most common way of detecting 2n pollen is looking for large pollen size since this implies the presence of 2n pollen in many genera (Van Laere et al, 2012). Some previous studies on Musa show that unreduced gametes do occur spontaneously in nature at low frequency (Dodds, 1943; Dodds and Simmonds, 1946; Sathiamoorthy and Balamohan, 1993)
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