Dissertation Research: Systematics of Harveya Hook. (Orobanchaceae) and the Evolution of Photosynthetic Loci in the Holoparasitic Genera Harveya and Hyobanche L.

哈维娅·胡克。(萝卜科)是一种原产于南非的寄生植物属。Harveya物种通过称为吸器的特殊根器官寄生其他植物。关于Harveya内物种的划分，物种之间的关系，或者Harveya物种是否特定于特定的寄主植物，人们知之甚少。这项研究的目标包括检查形态特征以确定物种的界限，根据形态特征和DNA序列推断系统发育历史，以及通过挖掘吸器来确定寄主专一性。在寄生植物中，编码光合蛋白的基因不受功能限制。这些基因可能带有功能减退突变。由于寄生植物从寄主植物中获得营养，而不是通过光合作用，这些突变不会损害适应性。因此，这些突变可能会遗传给后代，并在种群中固定下来。在不同谱系的寄生植物中，已经发现了光合作用基因的退化或缺失。另一种寄生虫Hyobance L.是Harveya的姊妹属。Hyobance携带基因rbcL的突变，这是一种叶绿体基因，编码Rubisco的大亚基，Rubisco是光合作用所必需的蛋白质。这些突变使rbcL在该属中不起作用。人们可能会认为Harveya会在这个基因中表现出类似的功能终止突变。然而，在到目前为止研究的所有Harveya物种中，rbcL编码一种功能蛋白。可以援引两个假设来解释这些突变的缺失。(A)Rubisco功能丧失是通过与Hyobance不同的途径在Harveya发生的。(B)Rubisco在Harveya保留功能，即兼性光合作用或未知的非光合作用作用。在这项研究中，编码小亚基(Rbcs)和大亚基(RbcL)的基因将在Hyobance和Harveya中进行测序，以比较其功能。此外，将对这些基因的RNA和蛋白质产物进行分析，以确定Rubisco功能在这些属中丢失的进化途径，或者Rubisco是否具有某些非光合作用功能。这项研究的重要性是多方面的。在同等大小的温带地区中，南非保持着最大的植物多样性。然而，随着人口的持续增长和人类栖息地向野生地区的扩张，许多物种可能会濒临灭绝。保护工作将依赖于对受威胁生物的进化史和生态需求(例如，合适的植物宿主)的了解。其次，Hyobance和Harveya的rbcL功能二分法为研究非光合作用植物的光合作用基因进化提供了一个理想的系统。此外，rbcL和rbcs具有完整的功能，但在植物的不同基因组中编码：rbcL在叶绿体基因组中，rbcL在细胞核中。其中一个基因的功能丧失可能会导致另一个基因的功能丧失。到目前为止，还没有研究检查一个基因组中发生的突变是否会反映到另一个基因组中。Hyobance和Harveya为研究这一现象提供了一个极好的框架。

Harveya Hook. (Orobanchaceae) is a genus of parasitic plants native to South Africa. Harveya species parasitize other plants by means of specialized root organs called haustoria. Little is known regarding delimitations of species within Harveya, relationships among species, or if Harveya species are specific to particular host plants. Goals of this study include the examination of morphological characters to determine species delimitations, the inference of phylogenetic history from morphological characters and DNA sequences, and determination of host specificity by excavation of haustoria. In parasitic plants, genes encoding photosynthetic proteins are not functionally constrained. These genes may bear function-reducing mutations. Because parasitic plants obtain nutrients from a host plant rather than through photosynthesis, these mutations are not detrimental to fitness. Therefore, these mutations may be passed to offspring and become fixed in populations. Deterioration or deletion of photosynthetic genes has been documented in parasitic plants of diverse lineage. Another parasite, Hyobanche L., is the sister genus of Harveya. Hyobanche bears mutations in the gene rbcL, a chloroplast gene encoding the large subunit of Rubisco, a protein necessary for photosynthesis. These mutations render rbcL non-functional in this genus. One might expect Harveya to demonstrate similar function-ending mutations in this gene. However, in all Harveya species examined thus far, rbcL encodes a functional protein. Two hypotheses may be invoked to explain the absence of these mutations. (a) The loss of Rubisco functionality has occurred via a different pathway in Harveya than in Hyobanche. (b) Rubisco retains function in Harveya, in facultative photosynthesis or an unknown, non-photosynthetic role. In this study, genes encoding both the small subunit (rbcS) and large subunit (rbcL) will be sequenced in Hyobanche and Harveya to compare functionality. Additionally, RNA and protein products of these genes will be assayed to determine the evolutionary pathway by which Rubisco functionality has been lost in these genera or if Rubisco serves some non-photosynthetic function. The importance of this study is several-fold. South Africa maintains the greatest floral diversity of any temperate region of equal size. However, many species may become endangered as population growth continues, and human habitations expand into wild areas. Conservation efforts will rely on knowledge of the evolutionary history and ecological needs (e.g. suitable plant hosts) of threatened organisms. Secondly, the dichotomy of rbcL functionality in Hyobanche and Harveya provides an ideal system for the study of evolution of photosynthetic genes in non-photosynthetic plants. Furthermore, rbcL and rbcS have an integrated function, but are coded in different genomes of the plant: rbcL in the chloroplast genome and rbcS in the nucleus. Loss of function of one of these genes may result in loss of function in the other. To date, no study has examined if mutations occurring in one genome may be mirrored by mutations in the other. Hyobanche and Harveya offer an excellent framework for investigating this phenomenon.