Multilocus half-tetrad analysis and centromere mapping in citrus: evidence of SDR mechanism for 2n megagametophyte production and partial chiasma interference in mandarin cv ‘Fortune’

Abstract

The genetic structure of 2n gametes and, particularly, the parental heterozygosity restitution at each locus depends on the meiotic process by which they originated, with firstdivision restitution and second-division restitution (SDR) being the two major mechanisms. The origin of 2n gametes in citrus is still controversial, although sexual polyploidisation is widely used for triploid seedless cultivar development. In this study, we report the analysis of 2n gametes of mandarin cv 'Fortune' by genotyping 171 triploid hybrids with 35 simple sequence repeat markers. The microsatellite DNA allele counting-peak ratios method for alleledosage evaluation proved highly efficient in segregating triploid progenies and allowed half-tetrad analysis (HTA) by inferring the 2n gamete allelic configuration. All 2n gametes arose from the female genitor. The observed maternal heterozygosity restitution varied between 10 and 82%, depending on the locus, thus SDR appears to be the mechanism underlying 2n gamete production in mandarin cv 'Fortune'. A new method to locate the centromere, based on the best fit between observed heterozygosity restitution within a linkage group and theoretical functions under either partial or no chiasmata interference hypotheses was successfully applied to linkage group II. The maximum value of heterozygosity restitution and the pattern of restitution along this linkage group would suggest there is partial chiasma interference. The implications of such a restitution mechanism for citrus breeding are discussed.

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

“…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. Cuenca et al [38] reported that maternal heterozygosity transmitted to each SDR 2n megagametophyte varied from 15.38% to 100%, with a mean value of 54.98%. In this study, based on all loci analyzed, the rate of maternal HR transmitted to each triploid hybrid ranged from 27.78% to 75.00%, with a mean of 51.46%.…”

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

“…Frost and Soost, (1968) and Kobayashi et al (1981) proposed that chromosome doubling in nucellar tissue might be the general mechanism underlying this process. It was confirmed by Aleza et al (2011). This characteristic could be under genetic control (Barrett and Hutchison, 1978) and/or affected by environmental conditions (Hutchison and Barrett, 1981;Aleza et al, 2011).…”

“…This characteristic can be a problem when apomictic genotypes are used as female parents in sexual hybridization. In this context, molecular marker techniques allow the differentiation between zygotic and nucellar seedlings and they have been developed and introduced within citrus breeding programs (Luro et al, 1995;Ruiz et al, 2000;Aleza et al, 2011). Nevertheless at practical level it is very difficult to recover large population of hybrids using apomictic genotypes as female parents.…”

“…They are often helpful for phylogenetic studies (Luro et al, 2001;Garcia-Lor et al, 2012;Garcia-Lor et al, 2013b), somatic hybrids characterization (Aleza et al, 2016b), discrimination between zygotic and nucellar seedlings (Ruiz et al, 2000;Ruiz and Asins, 2003), control of the origin of plants obtained by induced gynogenesis (Froelicher et al, 2007), molecular characterization of triploid cultivars (Cuenca et al, 2010), the analysis of the origin of unreduced gametes (Luro et al, 2004;Chen et al, 2008;Cuenca et al, 2011;Aleza et al, 2016a), mapping , and marker assisted selection in breeding . The homoplasic phenomena (identical allelic size arising from independent genetic events) observed by Barkley et al (2009) can limit the use of those markers for phylogenetic studies.…”

“…Thus, several research groups are working to diversify the tetraploid gene pool by identification and selection of spontaneous tetraploids (doubleddiploids; Aleza et al, 2009bAleza et al, , 2011) and somatic hybridization by protoplast fusion (Grosser et al, 2000;Ollitrault et al, 2007a;.…”