图2. 2n配子的形成机制
(A) 与体细胞相比正常配子的染色体数目减半; (B) 通过胞质融合或多核体形成的四倍体花粉母细胞产生PRD型2n配子, 如鸭茅(Falistocco et al., 1995); (C) 在减数分裂I期同源染色体无配对和分裂形成的FDR型2n配子将保留除重组片段外的所有亲本基因, 如雀稗(Filho et al., 2014)和多年生黑麦草(Chen et al., 1997); (D) 在减数分裂II期因分裂缺失形成的SDR型2n配子将保留除重组片段外比正常配子多1套染色体副本的2n配子, 如多年生黑麦草(Chen et al., 1997); (E) 在减数分裂I期部分染色体以姐妹染色单体分离, 其余以二价体分离, 随后在减数分裂II期分裂缺失形成IMR型2n配子, 如百合(Lim et al., 2001); (F) 减数分裂正常而进入配子体发育阶段时基因组加倍形成PMD型2n配子, 如马铃薯(Bastiaanssen et al., 1998)

Figure 2. Mechanisms of 2n gamete formation
(A) Chromosome numbers of normal gametes are halved compared with somatic cells; (B) PRD (pre-meiotic doubling) gametes are obtained as a result of the tetraploid pollen mother cells formed by cytomixis or syncytium, e.g., Dactylis glomerata (Falistocco et al., 1995); (C) In the FDR (first division restitution) type, pairing and division of homoeologous chromosomes do not occur during meiosis I, and the FDR 2n gametes maintain all of their parental genes except cross-over fragments, e.g., Paspalum jesuiticum (Filho et al., 2014) and Lolium perenne (Chen et al., 1997); (D) SDR (second division restitution) gametes with two copies of non-recombinant chromosomes are the result of the second division omission or cytokinesis abnormalities after normal anaphase II, e.g., L. perenne (Chen et al., 1997); (E) IMR (indeterminate meiotic restitution) gametes were discovered in Lilium longiflorum × Asiatic hybrid (Lim et al., 2001), in which some chromosomes are separated as univalents during meiosis I whereas the others are separated as bivalents, and subsequently the second division omission occurs during meiosis II; (F) PMD (post-meiotic doubling) gametes occurs due to the genome doubling of haploid spores during microgametogenesis, e.g., Solanum tuberosum (Bastiaanssen et al., 1998)