Flower Structure and Development of Spondias tuberosa and Tapirira guianensis (Spondioideae): Implications for the Evolution of the Unisexual Flowers and Pseudomonomery in Anacardiaceae

Abstract

Premise of research. Anacardiaceae comprise two subfamilies (Anacardioideae and Spondioideae) with small nectariferous flowers that are typically morphologically bisexual, whereas sometimes only the androecium or gynoecium is functional. In most Spondioideae, flowers are typically polysymmetric and obdiplostemonous, with as many antepetalous carpels as there are sepals (or petals), but frequently not all carpels are fertile, and the gynoecium can also be pseudomonomerous or more rarely monomerous. Pseudomonomery has received little or no attention in Spondioideae, and here we compare flowers of Spondias tuberosa, where all carpels form a locule but only one yields a fully developed ovule, and Tapirira guianensis, where only one carpel is fertile. Methodology. In these two species, we studied the flower structure and development using microtome serial sections, and light and scanning electron microscopy. Pivotal results. Both species share morphologically bisexual flowers with a similar floral bauplan and developmental pathway toward functionally male flowers. Their gynoecia share a syncarpous and entirely synascidiate ovary with the former center of the floral apex exposed between the free (and entirely plicate) styles and stigmas. In addition, the position of their single fertile locule varies in each flower and in S. tuberosa remains unclear until the development of the ovules. Conclusions. Functionally unisexual flowers and pseudomonomery likely evolved independently several times in Spondioideae and Anacardiaceae as a whole, and the structural differences between pseudomonomerous gynoecia in each subfamily are the results of heterochronic development. The evolutionary and functional significance of pseudomonomerous gynoecium in Anacardiaceae remains to be explored.