1,721,214 research outputs found
Systematics, phylogeny and reproductive biology of Mitrephora (Annonaceae)
No full textpublished_or_final_versionabstracttocEcology and BiodiversityDoctoralDoctor of Philosoph
Molecular phylogeny of the illiciales based on internal transcribed spacer sequences of ribosomal DNA
No full textpublished_or_final_versionEcology and BiodiversityDoctoralDoctor of Philosoph
Systematics and phylogeny of Dasymaschalon (Annonaceae)
No full textpublished_or_final_versionBiological SciencesDoctoralDoctor of Philosoph
Floral biology, pollination ecology and breeding systems of selected Dasymaschalon, Desmos, Pseuduvaria and Uvaria species (Annonaceae) inSouthern China and Australia
No full text The Annonaceae is a large, early-divergent family of angiosperms. Although the majority is pollinated by small beetles, there is a great diversity of floral morphologies. Evolutionary shifts of pollination mechanisms have occasionally been reported in disparate lineages including shifts to pollination by large beetles, flies, thrips, bees and cockroaches. It was previously hypothesized that floral morphological changes in different lineages are adaptive and correspond to evolutionary shifts of pollination mechanisms. This hypothesis is tested here by comparing selected species that have substantial morphological differences with their close relatives.
Comprehensive studies of the floral biology of four Annonaceae species, Dasymaschalon trichophorum, Desmos chinensis, Pseuduvaria mulgraveana and Uvaria cordata, are presented. All are beetle-pollinated. Dasymaschalon trichophorum, D. chinensis and U. cordata were inferred to be self-compatible based on inter-simple sequence repeat marker data as there was evidence of significant gene flow and a low level of genetic differentiation between populations. This was corroborated for D. chinensis and U. cordata by experimental controlled pollination tests for geitonogamy, in which both were shown to set fruit.
Similar floral phenological and pollination ecological results were obtained for D. chinensis and D. trichophorum despite significant differences in floral architecture, as the former has six petals (typical of most other Annonaceae species), whilst the latter only has three. The results suggested that the substantial change in floral morphology in these two closely-related genera is probably non-adaptive as there is no change in pollination system. It is hypothesised that the morphological change is likely due to the disruption of homeotic gene expression during floral organ development.
Studies of floral phenology and pollination ecology of Uvaria cordata revealed that it has a 3-day flowering rhythm and is pollinated by small beetles. The pollination ecology is unexpectedly similar to other species with typical beetle-pollination syndromes, although it lacks a pollination chamber and has pale-colored petals. It is suggested that species that lack a pollination chamber are more likely to be pollinated by guilds other than beetles or thrips as a floral chamber increases pollinator specificity. Several morphological and phenological characters are also presumably correlated with the evolution of generalist pollination to increase the pollination efficiency and assure fruit production, including torus shape, petal orientation, stigma shape, petal color, carpel and/or ovule number and the overlap of pistillate and staminate phases.
Pseuduvaria is unusual in the Annonaceae as the majority of species possess unisexual flowers. Most species were previously interpreted as having staminate and structurally pistillate flowers, with infertile staminodes in the latter. The ‘pistillate’ flowers of P. mulgraveana are shown to produce viable pollen, however, contradicting this hypothesis. It is therefore recommended that floral unisexuality in the genus be reassessed by testing more species from different clades.
Different strategies to promote xenogamy in the Annonaceae are reviewed, including protogyny, herkogamy, intra- and inter-individual phenological synchrony and dioecy. Three different mechanisms were recognized to achieve dioecy, including incomplete pollen development in hermaphroditic flowers, delayed anther dehiscence in hermaphroditic flowers and loss of androecium or gynoecium (with Pseuduvaria used as a paradigm).published_or_final_versionBiological SciencesDoctoralDoctor of Philosoph
Mating systems in sympatric Hedyotis species (Rubiaceae) in Hong Kong : the breakdown of distyly and its impact on hybridization and self-fertilization
No full textMany species in Hedyotis and related genera (Rubiaceae tribe Spermacoceae) exhibit
heterostyly, a floral polymorphism in which the anthers and stigmas occupy reciprocal
positions in two different ‘morphs’ within populations. This polymorphism is typically
associated with a biochemically mediated self-incompatibility system that prevents autogamy
and intra-morph fertilization. This study focuses on the reproductive biology, pollination
ecology and population genetics of the Hedyotis acutangula-shiuyingiae complex in Hong
Kong (comprising the often sympatric species H. acutangula, H. vachellii, H. shiuyingiae, H.
bodinieri and H. loganioides). Interspecific hybridization is investigated as a possible
explanation for the gene tree incongruence identified previously, with the breakdown of self-
and interspecific incompatibility hypothesized to have enabled hybridization within the
species complex. Correlations observed in self- and interspecific mate recognition systems
may be associated with unilateral incompatibility that acts as a barrier to hybridization: the
breakdown of specific self- and interspecific pollen recognition system might allow successful
interspecific mating. Hedyotis acutangula is demonstrated to be self- and interspecific
compatible, whereas H. vachellii, H. bodinieri and H. loganioides are self-compatible and
interspecific incompatible; H. shiuyingiae, in contrast, is strictly self-incompatible and
interspecific incompatible. The rapid growth of pollen tubes from heterospecific pollen in H.
acutangula indicates their ability to compete with conspecific pollen and undergo
interspecific hybridization. Population genetics data based on microsatellite markers provide
compelling evidence of the hybrid origin of H. bodinieri and H. loganioides and their
conspecificity. Reproductive biology and population genetic results confirmed the maternal
(H. acutangula) and paternal (H. shiuyingiae) parentage of these hybrids. Apis cerena, the
Asian honey bee, was identified as a pollinator, common to all the Hedyotis species studied.
The simultaneous flowering, overlapping anthesis and reproductive activity, shared
pollinators and interspecific pollen flow among sympatric Hedyotis species provide
compelling evidence for interspecific hybridization in the complex. Selection to avoid
interspecific hybridization can increase self-pollination in sympatric populations. The
functional breakdown of distyly and the presence of selfing syndromes in the sympatric
Hedyotis species studied suggests that self-pollination is favored, with anther dehiscence and
stigmatic receptivity prior to anthesis, self-compatibility and thrum morph-specific
spontaneous self-pollination in H. acutangula; self-incompatibility and relaxed stylar
dimorphism in H. vachellii; and stigmatic receptivity and anther dehiscence prior to anthesis
and high rates of spontaneous selfing in H. bodinieri. The success of self-pollination prior to
anthesis in H. bodinieri represents the first reported case of the co-occurrence of distyly and
pre-anthesis cleistogamy in Hedyotis. The functional breakdown of heterostyly and the
emergence of pre-anthesis cleistogamy represents an initial step in the evolutionary
transition from xenogamy to autogamy. The ability of cleistogamous pollen to compete with
inter-morph pollen provides the potential for pre-anthesis cleistogamy in H. bodinieri to break
the stability of distyly by reducing opportunities for outcrossing. Pre-anthesis cleistogamy is
likely a derived mechanism that ensures reproductive assurance despite potential mate and
pollinator limitation in the fragmented habitats in Hong Kong.published_or_final_versionBiological SciencesMasterMaster of Philosoph
Floral structure and reproductive biology of aegiceras (aegicerataceae), embelia (myrsinaceae) and maesa (maesaceae) in HongKong
No full textpublished_or_final_versionEcology and BiodiversityMasterMaster of Philosoph
Conservation genetics, phylogenetics, and photosynthetic pathway evolution in the Cirrhopetalum alliance (Bulbophyllum, orchidaceae) : testing hypotheses of loss of sex and slowdown of diversification
No full textOrchids offer rich opportunities for exploring evolutionary questions given their immense diversity and unique life history traits. In this thesis, I use species of the Cirrhopetalum alliance (Bulbophyllum, Orchidaceae) as a model system for understanding the processes and genetic mechanisms underlying the dynamics of orchid evolution at different scales, from within populations to among species.
Using an endangered epiphytic orchid, Bulbophyllum bicolor, as a model, I integrated an examination of population genetics with 12 microsatellite loci, taking into account a complex set of its life history traits (long-distance dispersal, self-incompatibility, pollinator-dependent sexual reproduction and clonal growth) to test for the hypothesis of ‘loss of sex’. My results identified just 22 multilocus genotypes among all 15 extant natural populations, with 12 of the populations found to be monoclonal and all three multiclonal ones exhibited a distinct phalanx clonal architecture. All populations were suggested to depend overwhelmingly on clonal growth for persistence, with a concomitant loss of sex due to an absence of pollinators and a lack of mating opportunities at virtually all sites, which are further entrenched by habitat fragmentation. Such cryptic life history impacts, potentially contributing to extinction debt, could be widespread among similarly fragmented, outcrossing tropical epiphytes, demanding urgent conservation attention.
A molecular phylogeny of the Cirrhopetalum alliance was reconstructed based on four DNA markers and a broad taxon sampling in tropical Asia, providing sufficient information for the recognition of the Cirrhopetalum alliance clade (CAC) as a well-supported monophyletic group, characterized by clear synapomorphies, following the exclusion of five putative Cirrhopetalum-allied sections, and the inclusion of sect. Desmosanthes. Most sections within the Cirrhopetalum alliance are demonstrated to be polyphyletic or paraphyletic, necessitating a new sectional classification. The inclusion of sect. Desmosanthes revolutionizes our understanding of the alliance, highlighting several significant evolutionary transitions in floral characters that are likely to represent adaptations to different pollination systems. The functional importance of the lateral sepals and labellum is revealed, with contrasting evolutionary patterns linked to different degrees of reproductive fitness: floral specialization (and thus speciation) might be driven through the costs of male fitness associated with the evolution of lateral sepals.
The dated phylogeny of CAC based on secondary calibration methods was applied to elucidate the impact of a controversial putative key innovation, Crassulacean acid metabolism (CAM) on the diversification of CAC. My results revealed that species diversification of CAC is positively correlated with the drop in atmospheric CO2 concentration (pCO2) in the late Miocene and a progressive slowdown of diversification afterwards. The evolution of CAM photosynthesis mirrors this pattern, with its origin coinciding with aridification and sharply decreased pCO2 in the late Miocene. CAM was revealed to be correlated with a ten-fold increase in extinction rate compared to C3 photosynthesis; strong asymmetrical transition between CAM and C3 was detected, both of which suggest CAM might represent an “evolutionary dead-end”, reflecting a failure to keep pace with the dynamics of pCO2 and contributing to the slowdown of diversification in CAC.published_or_final_versionBiological SciencesDoctoralDoctor of Philosoph
Molecular phylogenetics of Artabotrys (Annonaceae) : evaluating hypotheses of correlated evolution, historical biogeograhy and pollination system evolution
No full textAlthough Artabotrys is one of the most species-rich genera in the early divergent angiosperm family Annonaceae, with ca. 105 species of lianas distributed throughout the Palaeotropics, the genus has received relatively little research attention. The limited phylogenetic resolution and taxon sampling attained in previous studies have significantly constrained biogeographical and evolutionary inferences.
The phylogeny of Artabotrys was reconstructed in the present study based on 53 Artabotrys species (ca. 51% of species diversity), and four chloroplast (matK, ndhF, psbA-trnH and trnL-F) and 10 nuclear (AP3, At2g32520, GI, HMGS, LFY, MAG1, ncpGS, NIA, PHYA and RPB2) DNA regions, using maximum likelihood, maximum parsimony and Bayesian analyses. The results indicate that Artabotrys is unequivocally monophyletic, with a species-rich main Artabotrys clade (MAC) comprising distinct African and Asian sister clades, and an early divergent grade (EDG) comprising two African species.
Molecular divergence time estimation using a relaxed clock model and ancestral area and habitat reconstructions were performed to elucidate spatio-temporal diversification patterns. An ancestral range in Africa was inferred, with a single dispersal event to Asia. The most plausible explanation for the palaeotropical intercontinental disjunction involves overland migration across Arabia in the Miocene, prior to subsequent climate deterioration. The Late Miocene colonisation of Madagascar postdates the period conducive to rafting of vegetation mats from Africa to Madagascar. Dispersal across Wallace’s line occurred twice from the Late Miocene, when marine gaps diminished following convergence of the Sunda and Sahul shelves. Climatic niche conservatism is an underlying pattern in Artabotrys, with local niche shifts occurring rather recently.
Considerable variation in habitat and fruit and seed morphology in Artabotrys renders the genus ideal for evaluating correlated evolution between seed functional traits (seed size and physical defence) and various ecological correlates (climatic variables and dispersal-related traits). The relationships between functional traits and ecological correlates were examined using phylogenetic regression based on 43 Artabotrys species. The hypothesis that testa thickness and seed volume evolved towards distinct optima in lineages with different dispersal-related traits and precipitation regime was furthermore tested using model-fitting approaches. The two distinct trait optima inferred may correspond with two seed dispersal modes in Artabotrys: lineages with smooth testa and thin pericarp have evolved a thinner testa and smaller seeds, potentially reflecting dispersal by birds and primates with seed-spitting and seed-swallowing habits, whereas lineages with other combinations of testa texture and pericarp thickness have evolved a thicker testa and larger seeds, potentially reflecting dispersal by primates with more destructive mastication behaviour.
Comparative field studies on floral phenology and pollination ecology were undertaken in two exemplar species, viz. A. blumei from the MAC and A. brachypetalus from the EDG. The aberrant floral morphology of A. brachypetalus—in which all petals lack any distinction between blade and claw, and inner petals lack a projecting rim at the junction of the inner petal claw and blade—is associated with a generalist pollination system and the absence of a pollinator trapping mechanism. The firmly enclosed floral chamber is likely to be apomorphic for the MAC and associated with circadian pollinator trapping.published_or_final_versionBiological SciencesDoctoralDoctor of Philosoph
Systematics of Uvaria (Annonaceae) in Borneo and the reproductive biology of a Sri Lankan endemic, Uvaria semecarpifolia
No full textpublished_or_final_versionBiological SciencesDoctoralDoctor of Philosoph
Systematics of Desmos (Annonaceae) in Thailand, Peninsular Malaysia and Sumatra
No full textpublished_or_final_versionBiological SciencesMasterMaster of Philosoph
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