QTug.sau-3B is a major quantitative trait locus for wheat hexaploidization

Abstract

Meiotic nonreduction resulting in unreduced gametes is thought to be the predominant mechanism underlying allopolyploid formation in plants. Until now, however, its genetic base was largely unknown. The allohexaploid crop common wheat (Triticum aestivum L.), which originated from hybrids of T. turgidum L. with Aegilops tauschii Cosson, provides a model to address this issue. Our observations of meiosis in pollen mother cells from T. turgidum×Ae. tauschii hybrids indicated that first division restitution, which exhibited prolonged cell division during meiosis I, was responsible for unreduced gamete formation. A major quantitative trait locus (QTL) for this trait, named QTug.sau-3B, was detected on chromosome 3B in two T. turgidum×Ae. tauschii haploid populations. This QTL is situated between markers Xgwm285 and Xcfp1012 and covered a genetic distance of 1 cM in one population. QTug.sau-3B is a haploid-dependent QTL because it was not detected in doubled haploid populations. Comparative genome analysis indicated that this QTL was close to Ttam-3B, a collinear homolog of tam in wheat. Although the relationship between QTug.sau-3B and Ttam requires further study, high frequencies of unreduced gametes may be related to reduced expression of Ttam in wheat.

Keywords: CYCA1;2/TAM; Triticum aestivum; allopolyploidy; first division restitution; unreduced gametes.

Figure 1

Outline of the production of haploid and doubled haploid populations.

Figure 2

Chromosome constitutions. Genomic in situ hybridization on 42 root-tip chromosomes from A (pink), B (blue), and D (green) genomes (A). Fluorescence in situ hybridization using PAs1 (red), PSc119.2 (green), and PTa71 (yellow) as probes (B).

Figure 3

First division restitution (FDR) in LDN×AS60 F₁ hybrids. Twenty-one univalents are visible at early metaphase (A). Univalents aligned on the equator at metaphase (B). Sister chromatids starting to separate (C). Univalents not aligned on the equator when they begin to split into sister chromatids remain connected at the centromeres (D). A restitution nucleus formed (E) and chromosomes subsequently congregate on the equator (F). Chromosomes undergoing equational division at anaphase (G, H). Centromeres were labeled in green.

Figure 4

Final products of first division restitution (FDR) and standard meiotic division (SMD). FDR occurs in almost all PMCs of an anther from an LDN×AS60 hybrid (A) and produces dyads (B). Co-existence of FDR and SMD in an anther from an AS2255×AS60 hybrid (C) and resultant dyads and tetrads (D). Arrowheads in (D) indicate micronuclei. Adjacent cells (A) within an anther seem to be more synchronous than random cells (represented by red and green circles).

Figure 5

Asynchronous cell cycles from an anther of an AS2255×AS60 hybrid. Adjacent cells within an anther seem to be more synchronous (represented by red and green circles).

Figure 6

Frequency distribution and QTL for hybrid genome doubling. Frequency distribution of haploid plants with different seed setting rate for populations SynH1 (A) and SynH2 (C) and a QTL for the two haploid (F1) populations (B and D, red). This QTL was not detected in the doubled haploid (F₂) populations (B and D, green). Asterisk (*) after marker indicates deviation from the 1:1 expected segregation ratio at P < 0.05, ** at P < 0.01, *** at P < 0.005, **** at P < 0.001, and ****** at P < 0.0001.

Figure 7

Amino acid comparison of TAM/CYCA1:2 among wheat (Langdon-3A, 3B, and Chinese Spring-3D), rice (Os01g13260.1), Brachypodium (Bradi2g07946.1), and Arabidopsis (CYCA1:2/TAM). Red and green arrowheads indicate the two cyclin domains in Arabidopsis (Marchler-Bauer and Bryant 2004; Marchler-Bauer et al. 2009, 2011).

Figure 8

Expression changes of Ttam at different meiotic stages in the LDN×AS60 and AS2255×AS60 hybrids. Gene expression changes were assayed by qRT-PCR and analyzed by the 2^−ΔΔCt method. Wheat β-actin was used as the reference gene.