Tovar-Méndez Alejandro, Kumar Aruna, Kondo Katsuhiko, Ashford Amy, Baek You S, Welch Lillian, Bedinger Patricia A, McClure Bruce A
Division of Biochemistry, University of Missouri-Columbia, 117 Schweitzer Hall, Columbia, MO, 65211, USA.
Plant J. 2014 Mar;77(5):727-36. doi: 10.1111/tpj.12424. Epub 2014 Feb 11.
Interspecific reproductive barriers are poorly understood, but are central to the biological species concept. The pre-zygotic barriers between red- and green-fruited species in the tomato clade of the genus Solanum provide a model to better understand these barriers in plants. Compatibility usually follows the SI x SC rule: pollen from self-compatible (SC) red-fruited species is rejected on pistils of the predominantly self-incompatible (SI) green-fruited species, but the reciprocal crosses are compatible. This suggests that the interspecific reproductive barrier may be linked to the intraspecific SI mechanism. However, pollen from the SC red-fruited species is also rejected by SC accessions of green-fruited species that lack S-RNase, a key protein expressed in pistils of SI Solanum species. Thus, multiple mechanisms may contribute to the barrier between red- and green-fruited species. We tested whether an S-RNase-dependent barrier is sufficient for rejection of pollen from red-fruited species by introducing functional S-RNase, HT-A and HT-B genes from SI species into Solanum lycopersicum (cultivated tomato). We found that expressing S-RNase in combination with either HT-A or HT-B in the pistil is sufficient to cause rejection of pollen from all four red-fruited species. Thus, redundant mechanisms must operate side by side to prevent crosses between red- and green-fruited species in the clade, underlining the complexity of interspecific pollination barriers. Our results also have implications for mating system transitions. We suggest that these transitions must occur in a specific sequence, and that the transition from SI to SC also affects interspecific compatibility.
种间生殖障碍目前还知之甚少，但却是生物物种概念的核心。茄属番茄分支中红色和绿色果实物种之间的合子前障碍为更好地理解植物中的这些障碍提供了一个模型。兼容性通常遵循SI×SC规则：来自自交亲和（SC）红色果实物种的花粉在主要自交不亲和（SI）绿色果实物种的雌蕊上被拒绝，但反向杂交是兼容的。这表明种间生殖障碍可能与种内SI机制有关。然而，来自SC红色果实物种的花粉也会被缺乏S-RNase（一种在SI茄属物种雌蕊中表达的关键蛋白质）的绿色果实物种的SC材料拒绝。因此，多种机制可能导致红色和绿色果实物种之间的障碍。我们通过将来自SI物种的功能性S-RNase、HT-A和HT-B基因导入番茄（栽培番茄）来测试依赖S-RNase的障碍是否足以拒绝来自红色果实物种的花粉。我们发现，在雌蕊中同时表达S-RNase与HT-A或HT-B足以导致来自所有四种红色果实物种的花粉被拒绝。因此，冗余机制必须同时起作用，以防止该分支中红色和绿色果实物种之间的杂交，这突出了种间授粉障碍的复杂性。我们的结果也对交配系统转变有影响。我们认为这些转变必须按特定顺序发生，并且从SI到SC的转变也会影响种间兼容性。
