Phylogenetic relationships of Prosopis in South Africa: An assessment of the extent of hybridization, and the role of genome size and seed size in the invasion dynamics

Invasive alien plants have had diverse ecological and social impacts on recipient ecosystems and are a major problem for land managers. Successful management demands an understanding of the ecology of invading taxa. The invasive status and impacts are documented for Prosopis populations in South Africa. However, unresolved taxonomic issues, the extent of hybridization, the applicability of morphology as a species identification approach, and the role that some traits plays in the invasion success have not been studied. This creates a gap that hinders implementation of effective management policies. In this thesis I use a phylogenetic approach to determine the taxonomic make-up of invasive Prosopis populations in South Africa (Chapter 2) and compare the results to morphological identification (Chapter 3). I also look at seedling growth rates in the context of variation in genome size and seed size (Chapter 4). Almost all regions invaded by Prosopis are characterized by taxonomic uncertainty exacerbated by the ease of inter-specific hybridization. In Chapter 2 I aim to resolve taxonomic issues of invasive Prosopis populations in South Africa using a phylogenetic approach. In addition, I aim to unravel the extent of hybridization and the species involved in South Africa. Here, I found that Prosopis populations in South Africa comprise both reported and previously unreported species, indicating a need for a reassessment of the identity of invasive taxa. Hybridization is prevalent and all confirmed species are involved. These findings call for a rethink of legislation and management approaches, e.g. the selection of classical biological control agents. Overall the extent of hybridization indicates that Prosopis species in South Africa comprise a freely inter-breeding population typical of a syngameon. Proper morphological identification of invasive species is crucial for ecological studies and management of invasions. In Chapter 3, I use the total evidence approach to assess whether morphological approaches for identification are adequate for identifying Prosopis species in South Africa. I found that Prosopis taxa in South Africa cannot be reliably distinguished using existing morphological keys. This is likely due mainly to the proliferation of hybrids with a diverse morphology. Therefore, molecular tools are crucial for confirming any morphological identities and for determining the presence of any unreported species. Genome size and seed size have been reported to be associated with invasiveness in a number of plant groups, but not often in a system with multiple hybrids like Prosopis. In Chapter 4, I first investigate the relationship between genome size and seed size in invasive populations of Prosopis spp. in South Africa and secondly I investigate how genome and seed sizes influence germination and early growth. Here I found that genome size loses its distinctness, being diluted in hybridizing populations, but can still be used to assess hybridization events themselves. Large seed size seems to be important for invasiveness as it positively influences germination and early growth. This thesis confirms the taxonomic conundrum of Prosopis species in invasive ranges. This coupled with inadequacy of morphological identification calls for a global study involving native and invasive range taxa to clarify the existing confusions. In view of the presence of unreported Prosopis species in South Africa and extensive hybridization, a rethink of the current legislation and control is needed.外来入侵植物对受侵生态系统造成了多样化的生态与社会影响，是土地管理者面临的重大难题。高效的入侵管理有赖于对入侵类群生态学特性的深入认知。南非境内的牧豆树属（Prosopis）入侵种群的入侵状态与影响已有详实记载，但该类群存在的未解决分类学争议、杂交程度、形态学物种鉴定方法的适用性，以及部分功能性状在入侵成功中的作用，至今尚未得到系统研究。这一研究空白阻碍了高效入侵管理政策的落地实施。 本研究通过系统发育分析方法，明确南非入侵牧豆树种群的分类组成（第2章），并将其结果与形态学鉴定结果进行比对（第3章）；此外还探讨了基因组大小与种子大小变异对幼苗生长速率的影响（第4章）。几乎所有受牧豆树入侵的区域都存在分类学不确定性，而种间杂交的高发性进一步加剧了这一问题。 第2章中，本研究旨在通过系统发育分析方法解决南非入侵牧豆树种群的分类学争议，并阐明当地牧豆树种群的杂交程度及参与杂交的物种种类。研究发现，南非的牧豆树种群既包含已有文献记录的物种，也包含此前未被报道的新类群，这表明有必要重新评估入侵类群的分类身份。种间杂交现象普遍存在，且所有已确认的物种均参与其中。上述研究结果呼吁重新审视相关立法与管理策略——例如经典生物防治天敌的筛选工作。总体而言，杂交程度表明南非的牧豆树物种构成了一个自由杂交的种群，符合联种群（syngameon）的典型特征。 对入侵物种进行准确的形态学鉴定，对于生态学研究与入侵管理工作至关重要。第3章中，本研究采用总证据法，评估现有形态学鉴定方法是否足以区分南非的牧豆树物种。研究结果显示，依靠现有形态分类检索表无法可靠区分南非的牧豆树类群，这主要是由于形态多样的杂交个体大量存在所致。因此，分子生物学工具对于验证形态学鉴定结果、确认未被报道的新类群均具有关键作用。 已有研究表明，基因组大小与种子大小在诸多植物类群中与入侵性相关，但在牧豆树这类存在大量杂交个体的类群中，相关研究尚不多见。第4章中，本研究首先探讨了南非入侵牧豆树种群的基因组大小与种子大小之间的关联，其次分析了基因组大小与种子大小如何影响种子萌发及早期生长过程。研究发现，在杂交种群中基因组大小的差异会被稀释，从而失去区分度，但仍可用于评估杂交事件的发生情况。较大的种子尺寸似乎与入侵性密切相关，因为其对种子萌发及早期生长具有显著的正向影响。 本研究证实了入侵区域内牧豆树物种所面临的分类学难题。结合形态学鉴定方法的局限性，亟需开展涵盖原生境与入侵境类群的全球范围研究，以厘清当前的分类混乱问题。鉴于南非境内存在未被报道的牧豆树物种且杂交现象广泛，亟需重新审视现行的相关立法与防控策略。