Meiotic anomalies in hybrids between wheat and apomictic Elymus rectisetus (Nees in Lehm.) A. Love & Connor

Four [B.sub.III] hybrids (2n = 9x = 63, ABDStStYYWW) were obtained between wheat (Triticum aestivum L., 2n = 6x = 42, AABBDD) and a diplosporic and 2n-pollen-producing Elymus rectisetus (2n = 6x = 42, StStYYWW) accession. We determined whether apomeiosis and 2n pollen formation, which occur in the male parent, also occur in the Bill hybrids. Pistils were cleared for UV microscopy of callose and recleared for cytological observations using interference contrast microscopy. Callose fluorescence from most megaspore mother cells (MMCs) was typical of sexual species. These were classified as sexual. Abnormally elongated MMCs with walls devoid of callose were observed in 4.7% of pistils collected at meiotic prophase. These traits are typical of many apomeiotic MMCs in E. rectisetus. Asynapsis followed by mitosis in pollen mother cells (PMCs) consistently preceded unreduced pollen formation in E. rectisetus. This was not observed in the [B.sub.III] hybrids, though the following meiotic anomalies were observed: (i) asynapsis, aggregation of chromosomes to several poles, and multiple divisions to form four to eight unbalanced microspores, and (ii) incomplete synapsis, formation of anaphase I laggards, and a second division resulting in four unbalanced microspores with micronuclei. The wheat haplome enforced PMC meiotic pairing (albeit abnormal) among homologous E. rectisetus chromosomes and generally inhibited apomeiosis, also by enforcing meiosis. Diplospory may be caused by the expression of embryo sac signals from one genome precociously expressed with megasporogenesis signals from another. If this is correct, the successful induction of apomixis in wheat may require the transfer of alien gene cassettes that confer appropriate degrees of reproductive asynchrony.
The Australian wheatgrass Elymus rectisetus reproduces by facultative diplospory. During the early apomeiotic prophase, vacuoles accumulate in the chalazal region of the MMC, the nucleus moves to the center of [...]