Instituto de Investigaciones en Ciencias Agrarias de Rosario (IICAR), CONICET-UNR/Laboratorio de Biología Molecular, Facultad de Ciencias Agrarias, Universidad Nacional de Rosario. Campo Experimental Villarino, Zavalla, Provincia de Santa Fe, Argentina.
Instituto de Botánica del Nordeste (IBONE), CONICET-UNNE, Facultad de Ciencias Agrarias, Universidad Nacional del Nordeste, Casilla de Correo, Corrientes, Argentina.
Ann Bot. 2019 May 20;123(5):901-915. doi: 10.1093/aob/mcy228.
Apomixis is an asexual reproductive mode via seeds that generate maternal clonal progenies. Although apomixis in grasses is mainly expressed at the polyploid level, some natural diploid genotypes of Paspalum rufum produce aposporous embryo sacs in relatively high proportions and are even able to complete apomixis under specific conditions. However, despite the potential for apomixis, sexuality prevails in diploids, and apomixis expression is repressed for an as yet undetermind reason. Apomixis is thought to derive from a deregulation of one or a few components of the sexual pathway that could be triggered by polyploidy and/or hybridization. The objectives of this work were to characterize and compare the reproductive development and the timing of apospory initial (AI) emergence between diploid genotypes with potential for apomixis and facultative apomictic tetraploid cytotypes of P. rufum.
Reproductive characterization was performed by cytoembryological observations of cleared ovaries and anthers during all reproductive development steps and by quantitative evaluation of the ovule growth parameters.
Cytoembryological observations showed that in diploids, both female and male reproductive development is equally synchronized, but in tetraploids, megasporogenesis and early megagametogenesis are delayed with respect to microsporogenesis and early microgametogenesis. This delay was also seen when ovary growth was taken as a reference parameter. The analysis of the onset of AIs revealed that they emerge during different developmental periods depending on the ploidy level. In diploids, the AIs appeared along with the tetrad (or triad) of female meiocytes, but in tetraploids they appeared earlier, at the time of the megaspore mother cell. In both cytotypes, AIs can be seen even during megagametogenesis.
Overall observations reveal that female sexual reproductive development is delayed in tetraploids as compared with diploid genotypes, mainly at meiosis. In tetraploids, AIs appear at earlier sexual developmental stages than in diploids, and they accumulate up to the end of megasporogenesis. The longer extension of megasporogenesis in tetraploids could favour AI emergence and also apomixis success.
无融合生殖是一种通过种子产生母本克隆后代的无性生殖方式。尽管禾本科植物中的无融合生殖主要在多倍体水平上表达，但一些天然二倍体 Paspalum rufum 基因型以相对较高的比例产生无孢子胚囊，甚至在特定条件下能够完成无融合生殖。然而，尽管具有无融合生殖的潜力，二倍体仍以有性生殖为主，无融合生殖的表达受到未知原因的抑制。无融合生殖被认为源于性途径中一个或几个成分的失调，这种失调可能是由多倍体和/或杂交引起的。本研究的目的是描述和比较具有无融合生殖潜力的二倍体基因型和 P. rufum 的兼性无融合生殖四倍体细胞型之间的生殖发育和不定胚囊起始（AI）出现的时间。
通过对整个生殖发育阶段的澄清卵巢和花药进行细胞胚胎学观察，并对胚珠生长参数进行定量评估，对生殖特征进行了描述。
细胞胚胎学观察表明，在二倍体中，雌性和雄性生殖发育同样同步，但在四倍体中，大孢子发生和早期大配子发生相对于小孢子发生和早期小配子发生延迟。当以卵巢生长为参考参数时，也可以看到这种延迟。对 AI 出现时间的分析表明，它们的出现取决于倍性水平，出现在不同的发育时期。在二倍体中，AI 出现在雌性减数分裂小体的四分体（或三分体）期间，但在四倍体中，它们更早出现在大孢子母细胞时期。在这两种细胞型中，甚至在大配子发生期间也可以看到 AI。
总体观察表明，与二倍体基因型相比，四倍体中的雌性有性生殖发育延迟，主要在减数分裂时期。在四倍体中，AI 出现在比二倍体更早的有性发育阶段，并且在大孢子发生结束前积累。四倍体中大孢子发生的延长时间可能有利于 AI 的出现和无融合生殖的成功。
