Bicknell R A, Lambie S C, Butler R C
New Zealand Institute for Crop & Food Research Ltd, Christchurch, New Zealand.
Hereditas. 2003;138(1):11-20. doi: 10.1034/j.1601-5223.2003.01624.x.
Hieracium is an established model system for studying the cytological and genetic basis of gametophytic apomixis. In common with most known apomicts, the formation of 'maternal seed' is not exclusive in Hieracium, as apomixis operates in conjunction with a low level of sexuality. When this occurs the form of apomixis is described as 'facultative'. The formation of maternal seed in these plants is characterised by the avoidance of meiosis followed by the parthenogenetic development of an unreduced egg cell. In some ovules, however, meiosis does proceed, and sometimes the fertilisation of an egg cell presages embryogenesis. As a result, this mechanism of facultative apomixis leads to the formation of several different types of progeny, each representing a unique combination of meiosis/apomeiosis and fertilisation/parthenogenesis. Furthermore, fertilisation may involve either self or non-self pollen, leading to the recognition of six progeny classes from each individual plant. To facilitate an understanding of these processes we have developed a method for identifying individuals from different progeny classes based on the inheritance of introduced heterologous marker genes. This technique permits the screening of many thousands of seedlings at germination, and the consequent isolation of individuals associated with rare classes. Progeny profiles were determined for two apomictic accessions of Hieracium. Both were found to develop approximately 2.5% of their seed from meiotically derived eggs under the experimental conditions used and to have a rate of hybridity of approximately 2%. Evidence was also found for the action of a self-incompatibility mechanism operating in these plants despite the autonomous nature of apomixis in Hieracinum. As a demonstration of the utility of this approach, a study was conducted of polyembryony in one accession. The results indicate that there was a 7 fold greater likelihood that a meiotically derived seedling would arise in a polyembryonic seed than in a single-embryo seed. This indicates that facultative apomixis in Hieracium not only results from the simultaneous occurrence of sexual and asexual seed formation in the same capitulum as previously demonstrated, but most often as parallel processes within the same ovule.
山柳菊是研究配子体无融合生殖的细胞学和遗传基础的成熟模式系统。与大多数已知的无融合生殖植物一样，“母本种子”的形成在山柳菊中并非唯一现象，因为无融合生殖与低水平的有性生殖同时存在。当这种情况发生时，无融合生殖的形式被描述为“兼性”。这些植物中母本种子的形成特点是避免减数分裂，随后未减数的卵细胞进行孤雌生殖发育。然而，在一些胚珠中，减数分裂确实会发生，有时卵细胞受精预示着胚胎发生。因此，这种兼性无融合生殖机制导致形成几种不同类型的后代，每种后代代表减数分裂/无减数分裂和受精/孤雌生殖的独特组合。此外，受精可能涉及自花或异花花粉，从而从每株植物中识别出六个后代类别。为了便于理解这些过程，我们开发了一种基于导入的异源标记基因的遗传来识别不同后代类别的个体的方法。这项技术允许在种子萌发时筛选数千株幼苗，并随后分离出与稀有类别相关的个体。确定了山柳菊两个无融合生殖材料的后代谱系。在所使用的实验条件下，发现两者约2.5%的种子来自减数分裂产生的卵细胞，杂交率约为2%。还发现了这些植物中存在自交不亲和机制的证据，尽管山柳菊的无融合生殖具有自主性。作为这种方法实用性的一个例证，对一个材料中的多胚现象进行了研究。结果表明，多胚种子中减数分裂产生的幼苗出现的可能性比单胚种子大7倍。这表明山柳菊中的兼性无融合生殖不仅如先前所示是由于在同一个头状花序中同时发生有性和无性种子形成，而且最常见的是在同一个胚珠内作为并行过程。
