Gametophytic cross-incompatibility systems in corn have been the subject of genetic studies for more than a century. They have tremendous economic potential as a genetic mechanism for controlling fertilization without controlling pollination. Three major genetically distinct and functionally equivalent cross-incompatibility systems exist in Zea mays: Ga1, Tcb1, and Ga2. All three confer reproductive isolation between maize or teosinte varieties with different haplotypes at any one locus. These loci confer genetically separable functions to the silk and pollen: a female function that allows the silk to block fertilization by non-self-type pollen and a male function that overcomes the block of the female function from the same locus. Identification of some of these genes has shed light on the reproductive isolation they confer. The identification of both male and female factors as pectin methylesterases reveals the importance of pectin methylesterase activity in controlling the decision between pollen acceptance versus rejection, possibly by regulating the degree of methylesterification of the pollen tube cell wall. The appropriate level and spatial distribution of pectin methylesterification is critical for pollen tube growth and is affected by both pectin methylesterases and pectin methylesterase inhibitors. We present a molecular model that explains how cross-incompatibility systems may function that can be tested in Zea and uncharacterized cross-incompatibility systems. Molecular characterization of these loci in conjunction with further refinement of the underlying molecular and cellular mechanisms will allow researchers to bring new and powerful tools to bear on understanding reproductive isolation in Zea mays and related species.

玉米的配子植物交叉不相容系统已经成为遗传研究的一个多世纪了。它们作为控制施肥而不控制授粉的遗传机制具有巨大的经济潜力。玉米中存在三种主要的遗传上不同且功能相同的交叉不相容系统：Ga1，Tcb1和Ga2。这三者在任何一个位点都具有单倍型不同的玉米或端粒变种之间进行生殖隔离。这些基因座赋予蚕丝和花粉遗传上可分离的功能：雌性功能使蚕丝能够通过非自花粉阻止受精，而雄性功能则克服了同一位置的雌性功能障碍。对其中一些基因的鉴定为它们所提供的生殖分离提供了启示。将男性和女性因素都鉴定为果胶甲基酯酶揭示了果胶甲基酯酶活性在控制花粉接受与排斥之间的决定中的重要性，这可能是通过调节花粉管细胞壁的甲基酯化程度来实现的。果胶甲基酯化的适当水平和空间分布对于花粉管的生长至关重要，并受果胶甲基酯酶和果胶甲基酯酶抑制剂的影响。我们提供了一个分子模型，该模型解释了交叉不兼容系统如何运行，可以在Zea和未描述的交叉不兼容系统。这些基因座的分子特征与潜在分子和细胞机制的进一步完善相结合，将使研究人员能够带来新的强大工具，以了解玉米和相关物种的生殖分离。