Polyploidy in Fruit Tree Crops of the Genus Annona (Annonaceae).

Carolina Martin,José I Hormaza,Jorge Lora,Maria A Viruel

doi:10.3389/fpls.2019.00099

Carolina Martin, José I Hormaza + Show 2 more

Open Access

https://doi.org/10.3389/fpls.2019.00099

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Abstract

Genome duplication or polyploidy is one of the main factors of speciation in plants. It is especially frequent in hybrids and very valuable in many crops. The genus Annona belongs to the Annonaceae, a family that includes several fruit tree crops, such as cherimoya (Annona cherimola), sugar apple (Annona squamosa), their hybrid atemoya (A. cherimola × A. squamosa) or pawpaw (Asimina triloba). In this work, genome content was evaluated in several Annona species, A. triloba and atemoya. Surprisingly, while the hybrid atemoya has been reported as diploid, flow cytometry analysis of a progeny obtained from an interspecific cross between A. cherimola and A. squamosa showed an unusual ploidy variability that was also confirmed karyotype analysis. While the progeny from intraspecific crosses of A. cherimola showed polyploid genotypes that ranged from 2.5 to 33%, the hybrid atemoyas from the interspecific cross showed 35% of triploids from a total of 186 genotypes analyzed. With the aim of understanding the possible implications of the production of non-reduced gametes, pollen performance, pollen size and frequency distribution of pollen grains was quantified in the progeny of this cross and the parents. A large polymorphism in pollen grain size was found within the interspecific progeny with higher production of unreduced pollen in triploids (38%) than in diploids (29%). Moreover, using PCR amplification of selected microsatellite loci, while 13.7% of the pollen grains from the diploids showed two alleles, 41.28% of the grains from the triploids amplified two alleles and 5.63% showed up to three alleles. This suggests that the larger pollen grains could correspond to diploid and, in a lower frequency, to triploid pollen. Pollen performance was also affected with lower pollen germination in the hybrid triploids than in both diploid parents. The results confirm a higher percentage of polyploids in the interspecific cross, affecting pollen grain size and pollen performance. The occurrence of unreduced gametes in A. cherimola, A. squamosa and their interspecific progeny that may result in abnormalities of ploidy such as the triploids and tetraploids observed in this study, opens an interesting opportunity to study polyploidy in Annonaceae.

Highlights

Polyploidy is believed to be a major mechanism of adaptation and speciation, recognized as a major force in evolution (Van de Peer et al, 2017) and very valuable for crop improvement (Udall and Wendel, 2006; Mason, 2016)
Different theoretical models of polyploidy have been elaborated considering that the success of tetraploids arisen by sexual polyploidization within a diploid population is influenced by the frequency with which unreduced gametes are produced by diploids (Felber, 1991; Felber and Bever, 1997; Ramsey and Schemske, 1998; Li et al, 2004)
A. cherimola, A. squamosa, the diploid atemoyas observed by flow cytometry analysis and A. muricata showed 14 somatic chromosomes (Figure 2)

Summary

Introduction

Polyploidy is believed to be a major mechanism of adaptation and speciation, recognized as a major force in evolution (Van de Peer et al, 2017) and very valuable for crop improvement (Udall and Wendel, 2006; Mason, 2016). Studies on polyploid evolution have revealed that the most common mechanism of polyploidy in flowering plants involve unreduced gametes (Bretagnolle and Thompson; Darlington, 1937, 1965; Harlan and deWet, 1975; deWet, 1979; Karpechenko, 2010; Kreiner et al, 2017). There are two main ways to produce unreduced diploid gametes in plants, as a result of first-division restitution (FDR) or second-division restitution (SDR) during meiosis (Hermsen, 1984; De Storme and Geelen, 2013). In FDR, the first meiotic division fails and, as consequence, the two chromosomes of the unreduced gamete are non-sister chromatids. In SDR, the second meiotic division fails and the two chromosomes of the unreduced gamete are sister chromatids. A third proposed mechanism has been reported as indeterminate meiotic restitution (IMR), which produces microspores with disproportionate number of chromosomes due to a restitution mechanism (Lim et al, 2001; De Storme and Geelen, 2013)

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