Instituto de Biología, Universidad de Antioquia, AA 1226, Medellín, Colombia.
Universidad Nacional de Colombia, Sede Bogotá, Facultad de Ciencias, Instituto de Ciencias Naturales, AA 7495, Bogotá, Colombia.
Am J Bot. 2021 Aug;108(8):1315-1330. doi: 10.1002/ajb2.1719. Epub 2021 Aug 30.
Floral spurs are key innovations associated with elaborate pollination mechanisms that have evolved independently several times across angiosperms. Spur formation can shift the floral symmetry from radial to bilateral, as it is the case in Tropaeolum, the only member of the Brassicales with floral nectar spurs. The genetic mechanisms underlying both spur and bilateral symmetry in the family have not yet been investigated.
We studied flower development and morphoanatomy of Tropaeolum longifolium. We also generated a reference transcriptome and isolated all candidate genes involved in adaxial-abaxial differential growth during spur formation. Finally, we evaluated the evolution of the targeted genes across Brassicales and examined their expression in dissected floral parts.
Five sepals initiate spirally, followed by five petals alternate to the sepals, five antesepalous stamens, three antepetalous stamens, and three carpels. Intercalary growth at the common base of sepals and petals forms a floral tube. The spur is an outgrowth from the adaxial region of the tube, lined up with the medial sepal. We identified Tropaeolum specific duplications in the TCP3/4L and STM gene lineages, which are critical for spur formation in other taxa. In addition, we found that TM6 (MADS-box), RL2 (RAD-like7), and KN2/6L2 and OSH6L (KNOX1 genes), have been lost in core Brassicales but retained in Tropaeolum.
Three genes are pivotal during the extreme adaxial-abaxial asymmetry of the floral tube, namely, TlTCP4L2 restricted to the adaxial side where the spur is formed, and TlTCP12 and TlSTM1 to the abaxial side, lacking a spur.
花距是与精心设计的传粉机制相关的关键创新，这些机制在被子植物中已经独立进化了多次。花距的形成可以使花的对称性从辐射对称转变为两侧对称，就像 Brassicales 目中唯一具有花蜜花距的 Tropaeolum 属那样。目前尚未研究该科中花距和两侧对称的遗传机制。
我们研究了 Tropaeolum longifolium 的花发育和形态解剖学。我们还生成了参考转录组，并分离了在花距形成过程中涉及背腹侧差异生长的所有候选基因。最后，我们评估了目标基因在 Brassicales 目中的进化，并在分离的花部分中检测了它们的表达。
五个萼片呈螺旋状起始，随后是五个与萼片交替的花瓣，五个萼片外轮雄蕊，三个花瓣外轮雄蕊，和三个心皮。萼片和花瓣的共同基部的居间生长形成花管。花距是从管的腹侧区域向外生长的，与中萼片对齐。我们在 TCP3/4L 和 STM 基因谱系中鉴定出 Tropaeolum 特有的重复，这些基因在其他类群中对于花距的形成至关重要。此外，我们发现 TM6（MADS 盒）、RL2（RAD-like7）、KN2/6L2 和 OSH6L（KNOX1 基因）在核心 Brassicales 中丢失，但在 Tropaeolum 中保留。
在花管的极端腹背侧不对称中，有三个基因至关重要，即仅在形成花距的腹侧侧 TlTCP4L2，以及在缺少花距的背侧侧 TlTCP12 和 TlSTM1。
