Diversity and Evolution of the Multicellular Protuberances in Cuscuta (Convolvulaceae) and the Function of the Stomatiferous Protuberances in Cuscuta subgenus Grammica
Cuscuta (Convolvulaceae), also known as the dodders, is a holoparasitic genus comprised of ca. 200 species grouped into four subgenera: Monogynella, Cuscuta, Pachystigma, and Grammica. The presence of unique multicellular structures, referred to as stomatiferous protuberances (SPs), was reported on the stems of subgenus Grammica over a century ago and was forgotten until similar SPs were observed on the flowers of several new Grammica species. The stems and flowers were examined in 136 Cuscuta taxa, and SPs were discovered on all of the haustorial stems of the species in the subgenus Grammica, as well as on the perianth of subgenera Cuscuta and Pachystigma. Other multicellular structures, referred to as extrafloral nectaries (EFNs), found in the subgenus Monogynella, differ both morphologically and functionally from the SPs. The diversity and evolution of the multicellular protuberances of Cuscuta (including both EFNs and SPs), as well as the function of the SPs are explored throughout this thesis. A morphological survey of both stems and flowers in Cuscuta was performed and an examination of the protuberances was completed using light and scanning electron microscopy. Three distinct morphological forms of SPs in Grammica were found: dome-like, conical to cylindrical, and crest-like, and in subgenera Cuscuta and Pachystigma SPs were found with a “diffuse” structure. Each protuberance possesses one or several distal stomata. Species in the subgenus Grammica developed two functional types of stems: exploratory stems with no SPs, and haustorial stems with numerous SPs. Using two parasite/host systems in the field, C. gronovii/Solidago canadensis in Canada, and C. costaricensis/Tithonia tubiformis in Mexico, stomatal conductance and water uptake were determined. Stomatal conductance rates were higher in hosts that were not parasitized by Cuscuta, suggesting hosts’ water preservation when Cuscuta is present. Haustorial stems, with SPs, had a higher stomatal conductance rate than the exploratory stems that are without SPs. This pattern was found in both parasite-host systems. The water uptake of parasitized hosts was significantly higher than non-parasitized hosts and can be explained in part by the water loss at the level of the SPs. Furthermore, Cuscuta species within the subgenus Grammica, with floral SPs, grow in arid areas (average precipitation less or equal to 90mm) during flowering/fruiting, which suggest that SPs may have evolved to stimulate this water uptake through the host during these stages. Within this thesis, the development of two functional types of stems in the subgenus Grammica is discussed for the first time, as well as the evolution of floral SPs across the genus. The flowers of Cuscuta are important, as they are the only way to distinguish the ca. 200 species from one another. The documentation of the floral SPs throughout the genus can help with the identification of some of the species. Furthermore, the function of the SPs, which has not been discussed in previous literature, is also studied in this thesis for the first time
