Phylogenetic relationships based on RAD-seq datasets highlights the necessity for a taxonomic revision in Hawaiian Melicope. Of the five informally used Stone’s sections in the lineage, I only could confirm only two as monophyletic groups, the former genus Platydesma and Stone’s section Pelea (Figure 3.2). Considering the
distinctiveness of the Platydesma lineage, especially the palmoid habit,
hermaphroditic flowers and staminal tube (Wagner et al., 1999b) and its early divergence (Figure 4.1), the group should receive formal recognition. The three remaining Stone’s sections, which comprise the vast majority of the lineage are each non-monophyletic. Stone’s sections Cubicarpa and Megacarpa are paraphyletic to each other and all taxa belonging to either were resolved in clade I with no apparent pattern to the divergence between the two fruit morphologies (Figure 3.2). The main characteristic delimitating the two groups is the degree of carpel connation, which is described as “up to 2/3 of their length” for Megacarpa and “nearly to completely” for Cubicarpa (Stone et al., 1999). The results I present in chapter 3 suggest the separation between these two Stone’s sections is artificial and they should be merged as carpel connation seems a continuous character rather than two discrete states. Stone’s section Apocarpa is divided into two different lineages with the majority of taxa resolved in clade Apocarpa 1 (clade III, Figure 3.2) while only three species included herein comprise the Apocarpa 2 clade (clade II, Figure 3.2). These three species do share some morphological traits, noticeably the sprawling, shrubby habit (except M.
stonei), a glabrous exo- and endocarp and the few-flowered inflorescences.
Additionally, these three taxa occur exclusively in mesic habitats (Stone et al., 1999;
Wood et al., 2017). However, neither one of these characteristics, nor their
combination, is unique to this lineage. Further studies of the characteristics of all Apocarpa species are required to identify trait patterns characterizing these clades.
These research efforts need to include samples of M. elliptica, which I could not sample for my thesis. M. elliptica represents the type species of Stone’s section Apocarpa. Results from Sanger sequencing suggest the species represents a third apocarpous lineage within the Hawaiian Melicope and may be closely related to clade I (Appelhans et al., 2014b). Price and Wagner (2004) hypothesized that the
individuals of M. elliptica might represent hybrids, as they grew near several species of clade I. Inclusion of M. elliptica samples is required to determine, whether one of the two Apocarpa clades resolved will have to be renamed or even both. The revision of the lineage must also necessarily include the Marquesan species of Melicope. My results confirm previous results suggesting that the Hawaiian Islands are the origin of two independent colonization events to the Marquesas Islands, one resulting in a local adaptive radiation of seven species (Hartley, 2001; Appelhans et al., 2018b) represented here by M. hivaoaensis (clade I; Figure 3.2). Inclusion of these altogether eight Marquesan species in further research is necessary to conclusively identify phenotypic traits characterizing clades in Hawaiian Melicope.
Some species require taxonomic revision as well. In this thesis, I sampled 84% of the 54 currently described Hawaiian species, 24 of which I could include with multiple samples. Of these 24 multi-sampled species four were resolved as non-monophyletic:
M. clusiifolia, M. haupuensis, M. knudsenii, and the M. feddei. Melicope knudsenii is the only species in the entire lineage occurring on non-neighboring islands, Kauaʻi and Maui (Stone et al., 1999). Previous phylogenetic efforts in Hawaiian Melicope
provided evidence that the populations of M. knudsenii actually represent up to three distinct species (Appelhans et al., 2014b). Subsequently, one population occurring on the island of Kauaʻi was described as a new species (Wood et al., 2017). In this thesis, I could confirm that the populations of M. knudsenii on Kauaʻi and Maui represent two distinct taxa as indicated in Appelhans et al. (2014b). The individuals are all members of clade III, with the Maui populations sister to M. hawaiensis and the populations from the type location on Kauaʻi type population sister to M. barbigera.
Either relationship receives high statistical support (Figure 3.3). However, the results of quartet sampling indicate the possibility of reticulate evolution in the Maui
population, possibly involving M. barbigera (Figure 3.3). This could result in the reconstructed close relationship between the two taxa. Further studies are required to definitively elucidate the relationship of the Maui population of M. knudsenii before it can be resurrected as a separate species during taxonomic revision.
Melicope haupuensis is polyphyletic (clade III, Figure 3.2) and the only sample in my thesis with incongruent constellations among the individuals based on different
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datasets (Figure 3.2, Supplemental Figures 3.1-3.4). Several individuals were
observed in the field showing intermediate phenotypes between M. haupuensis and M. barbigera (personal observation K. Wood), which might be of hybrid origin. The results of quartet sampling support the indication of extensive reticulate evolution, as several nodes show skewed discord (Figure 3.3; Pease et al., 2018). However, the taxon sampling in my thesis is not sufficient to address this question further. A
thorough population-level sampling of the taxa in question, including M. knudsenii as sister to M. barbigera, will be required to address the possibility, extent, and direction of horizontal gene transfer in this clade.
Finally, M. clusiifolia is paraphyletic with respect to both M. haleakalae and M.
waialealae (Figure 3.2). Melicope clusiifolia is the most widespread species in the entire lineage, occurring on all main islands, and characterized by large morphological plasticity. The most recent taxonomic treatment synonymized all previous attempts at subdividing this species (St. John, 1944; Stone, 1969) considering the variability to represent continuously varying rather than discrete characters. In the same spirit, M.
haleakalae, differing from M. clusiifolia mainly by persistent sepals and M. waialealae differing mainly in leaf shape (Stone et al., 1999) might be considered representing further morphological variability. Thus, Stone’s section Pelea might only comprise one highly plastic species. On the other hand, the relationships of these three taxa might indicate speciation in progress. In this case, the deep nesting of M. haleakalae within M. clusiifolia would represent a case of progenitor-derivative-speciation
(Crawford, 2010). Finally, there is a clear geographical signal for the diversification in this group (Figure 4.2), which would indicate limited gene flow among islands.
Extensive sampling of populations of Stone’s section Pelea and subsequent
morphological and molecular research will be required to definitively elucidate the taxonomy of this clade.
M. wawraeana might be the most cryptic taxon within the lineage. My sampling included two individuals from Kauaʻi that correspond closely, but not entirely to the type-morphology of the species. The core populations of the species are situated on Oʻahu and show some degree of morphological variability. In “non-core”
populations this variability is extended further and resulted in synonymization of several species names (Stone et al., 1999). However, one of these synonyms, M.
hiiakae, was tentatively resurrected by Hartley and Stone (1989) and treated as a separate species ever since (Wagner et al., 1999a; Imada et al., 2011; Wood et al., 2016). My results provide evidence for this resurrection as the taxon is resolved as sister to M. christophersenii in clade I and not closely related to the two wawraeana-like individuals (Figure 3.2). This indicates that other non-core populations might
represent different taxa as well. The M. wawraeana-like populations from Kauaʻi sampled herein are cryptic as well. One of these M. sp. (specimen KW17111) is
resolved in a monophyletic clade with the individuals representing M. feddei (Figure 3.2). This indicates that at least one of the populations from Kauaʻi might actually represent M. feddei instead of M. wawraeana. The second M. sp. (specimen KW15733) is closely related to M. kavaiensis and both taxa show rogue behavior related to the Marquesan sample M. hivaoaensis (Figure 3.3, Supplemental Figures 3.1-3.4), which might reflect a hybridization event. In addition to putative hybridization events in the M. wawraeana complex (chapter 3, Imada et al., 2011), there is also evidence for polyploidy in some individuals (Guerra, 1984). Further detailed morphological, molecular and karyological studies are required to determine the status and evolutionary history of this potentially artificial taxon (Stone et al., 1999).
Finally, the sampling also included two species with samples corresponding to both, the type morphology and deviating morphotypes, M. barbigera and M. ovata. The deviating morphotypes are resolved as monophyletic sister groups to the samples representing the type morphology, which are also resolved as monophyletic (Figure 3.2). This again indicates speciation including phenotypic changes. Further studies are required to ascertain whether the speciation events are in process or sufficiently advanced to recognize these deviating morphotypes as separate species or
subspecies.