Comparative embryology ofStifftiaandWunderlichiaand implications for its evolution in Asteraceae

Abstract

Asteraceae are one of the largest families of angiosperms and comprise > 1700 genera and 24000 species currently in 13 subfamilies. However, despite recent and continuous improvement regarding their classification, little is known about their embryological diversity, especially in some of the early-diverging subfamilies including Stifftioideae and Wunderlichioideae. These subfamilies were previously considered to be similar, sharing several external features such as anther appendages, pollen grains, pappus bristles and cypsela trichomes. Ontogenetic and anatomical studies could reveal deeper levels of homology and expand the features of interest for the systematics of the family. Also, the lack of knowledge of the early-divergent subfamilies precludes the reconstruction of evolutionary trends in Asteraceae and Asterales. Here, we compared the anthers, ovaries, ovules, fruits and seeds of two Stifftia spp. and two Wunderlichia spp. to identify possible evolutionary trends. Our results show that embryology in these genera is similar, with slight differences in pollen grain ornamentation, types and distribution of ovary trichomes, antipodal persistence, pericarp tissue organization, occurrence of phytomelanin in the fruit wall, the form of the carpopodium, shape and lignification of carpopodial cells, form and distribution of pappus bristles, and lignification patterns of exotesta cells. The results also demonstrate novelties for the family, including Basic-type anther wall development and trichomes on the inner ovary epidermis of W. mirabilis, funicular trichomes in both Stifftia spp., and funicular stomata and phytomelanin in the pericarp of both Wunderlichia spp. Therefore, our data support Stifftioideae and Wunderlichioideae as distinct but closely related subfamilies. The results also open new perspectives on how anther, ovary and fruit evolved in the family. The occurrence of a Basic-type anther wall in early-divergent subfamilies may represent a symplesiomorphy or apomorphy for the family. This will depend on how the anther develops in the subfamilies related to Wunderlichioideae and Stifftioideae. Vascularization of the ovary was different from that previously known in the family and throws doubts upon its origin. Phytomelanin is a rare substance in plants, found only in seeds of most families of Asparagales and fruits of some Asteraceae; it had previously been found in fruits of Heterocoma (Cichorioideae) and some Asteroideae. We detected phytomelanin in Wunderlichia, indicating that presence of this substance has evolved independently three times in the fruits of the family.