2n megagametophyte formed via SDR contributes to tetraploidization in polyembryonic ‘Nadorcott’ tangor crossed by citrus allotetraploids

Abstract

2 n megagametophyte formation plays an important role in polyploidization in polyembryonic citrus and is valuable for plant improvement. Tetraploid plants are frequently observed in the seedlings of diploid polyembryonic citrus genotypes. However, the mechanisms underlying the formation of tetraploids are still indistinct when apomictic citrus genotypes are used as female parent to cross with tetraploids. Herein, 54 tetraploid progenies, which were unexpectedly obtained previously from four 2x × 4x crosses using polyembryonic 'Nadorcott' tangor as seed parent, were analyzed by 22 simple sequence repeat (SSR) markers, aiming to reveal their genetic origin and the mechanism underlying 2n megagametophyte formation. The results showed that 13 tetraploids from all these four crosses were doubled diploids as indicated by their identical SSR allelic profile with their female parent; while the remaining 41 tetraploids apparently exhibited paternally derived alleles, which confirmed their zygotic origin. Furthermore, the genotyping of all hybrids indicated that all of them arose from 2n megagametophytes. Based on the genotypes of 2n megagametophytes, the analysis of maternal heterozygosity restitution (HR) for each marker showed that it varied from 0.00 to 87.80 % with a mean value of 40.89 %. In addition, it was observed that 13 markers displayed a lower rate than 50 %. On the basis of the above results, it can be speculated that the second division restitution (SDR) is the mechanism underlying the 2n megagametophyte formation in 'Nadorcott' tangor. The elucidation of the mechanism of 2n megagametophyte formation will be of great help to optimize further sexual hybridization for polyploids in citrus.

Male Parent Identification of Triploid Rubber Trees (Hevea brasiliensis) and the Mechanism of 2n Gametes Formation

“…For example, high temperature can induce 2n female gametes in Populus [39,40]. However, gamete formation is controlled mainly by genes [32,41]. Some genes, such as AtPS1 [42] and JASON [43], were reported to be involved in 2n gamete formation in Arabidopsis.…”

“…Xie et al [32] reported that SDR was the mechanism of 2n megagametophyte formation in 'Nadorcott' tangor, because the rate of maternal HR varied from 0.00% to 87.80% for 22 SSR markers, with 13 exhibiting a heterozygosity rate <50%. In the present study, the rate of maternal HR of all 25 markers varied from 0% to 100%, with a heterozygosity rate <50% for 10 markers, suggesting that SDR may be the mechanism underlying 2n megagametophyte formation.…”

“…The genotypes of 2n gametes were directly read when parental alleles displayed as completely different based on the allele number and type. The rate of maternal HR (HR%) was estimated as described by Xie et al [32], calculated as: HR% = n ab /(n ab + n aa + n bb ) × 100…”

Genetic Structure of Diploid Gametes for the Production of Triploid Citrus Hybrids

“…Pérez-Tornero et al (2012) obtained triploid hybrids at a frequency of 5.8% to 8.6% in hybridizations between diploid plants of `Verna´ as the female parent and `Fino´ as the male parent. In mandarins, greater differences between genotypes have been observed, ranging from less than 1% for clementines to over 22% for `Sukega´ and `Ortanique´ tangor Aleza et al, 2010b;Wakana et al, 1982;Esen and Soost, 1971;Xie et al, 2014).…”

“…We have observed the same result in eight over the 72 tetraploid recovered plants. Xie et al (2014) also recovered tetraploid plants from the reciprocal hybridization, 2x x 4x, and tetraploid plants were originated as a consequence of 2n female gametes. Here, we have obtained a high number of tetraploid hybrids from 2n pollen gametes (64 over 72 tetraploid recovered plants from only 50 and 32 collected fruits of MCSO and FCSO respectively).…”