Natural hybridization is reproduction (without artificial influence) between two or more species/populations which are distinguishable from each other by heritable characters. Natural hybridizations among marine fishes were highly underappreciated due to limited research effort; it seems that this phenomenon occurs more often than is commonly recognized. As hybridization plays an important role in biodiversity processes in the marine environment, detecting hybridization events and investigating hybridization is important to understand and protect biodiversity. The first chapter sets the framework for this disseration study. The Cohesion Species Concept was selected as the working definition of a species for this study as it can handle marine fish hybridization events. The concept does not require restrictive species boundaries. A general history and background of natural hybridization in marine fishes is reviewed during in chapter as well. Four marine fish hybridization cases were examed and documented in Chapters 2 to 5. In each case study, at least one diagnostic nuclear marker, screened from among ~14 candidate markers, was found to discriminate the putative hybridizing parent species. To further investigate genetic evidence to support the hybrid status for each hybrid offspring in each case, haploweb analysis on diagnostic markers (nuclear and/or mitochondrial) and the DAPC/PCA analysis on microsatellite data were used. By combining the genetic evidences, morphological traits, and ecological observations together, the potential reasons that triggered each hybridization events and the potential genetic/ecology effects could be discussed. In the last chapter, sequences from 82 pairs of hybridizing parents species (for which COI barcoding sequences were available either on GenBank or in our lab) were collected. By comparing the COI fragment p-distance between each hybridizing parent species, some general questions about marine fish hybridization were discussed: Is there any correlation between genetic similarity and the potential for hybridization in marine fishes? In some particular geographic locations that have the existence of several different hybridization reports, are the species involved in hybridization among those reports all closely related or distantly related? Can any associations between parent species’ similarities and hybrid spots be found?

The morphological characters, pollen viability, and molecular markers are used in this study to assess the inter-species differentiation in Taiwan Trema( T. orientalis, T. tomentosa, T. cannabina, and the hybrid ). The hybrid was proposed to have been originated from T. tomentosa and T. cannabina natural hybridization with morphological, pollen viability and molecular marker evidences. The four taxa are variable in the following morphological characters: growth form, terminal bud color, leaf size, leaf apex, leaf base, leaf vestiture, leaf texture, leaf nerves, petiole, stipule size, inflorescence sex, male and female inflorescence length, flower number, mature fruit color, perianth vestiture, male flower perianth size, pistillode size, pistillode shape, filament length, and female flower size. The author also found differences in leaf shape, leaf size, leaf base, and leaf vestiture between adult and juvenile individuals of T. orientalis are ontogenetical variations. The hybrid is morphological intermediate between T. tomentosa and T. cannabina, possessing species-specific morphological characters of either species. Leaf trichome morphology was observed under scanning electronic microscope, and a unicellular trichome type with bulbous base, smooth surface, and creeping looking, is specific to T. orientalis leaf abaxial surface. In pollen viability tests, the hybrid had the lowest average pollen stainbility among tour taxa, but varied widely from 48.5 % to 81.6 %. In additivity test of molecular markers, for all 8 species-specific molecular markers of T. tomentosa, 6 were detected in hybrid; for all 14 species-specific molecular markers of T. cannabina, 11 were detected in hybrid; and none of 14 species-specific molecular markers of T. orientalis were detected in hybrid. Additionally, there were some recessive homozygote alleles detected in hybrid molecular marker, and even missing in T. tomentosa molecular markers. According to this evidence, there was a possible introgression between the hybrid and parental species-T. cannabina. In similarity dendrogram derived from molecular markers, all samples were clustered into four taxa-corresponded groups, the hybrid was placed between T. tomentosa and T. cannabina, and closely related to T. cannabina.

La hibridació natural a plantes és un fenòmen àmplement conegut. És una important font de variabilitat que accelera l'evolució de les espècies. Es creu que és l'origen de moltes angiospermes, entre elles endemismes locals. Per altra banda, també pot tenir efectes negatius per la supervivència d'aquests endemismes, diluint els seus trets direfencials. En aquesta tesi s'estudia la possible hibridació natural que afecta a tres endemismes baleàrics (Viola jaubertiana, Lotus fulgurans i Helichrysum crassifolium), des del punt de vista dels marcadors moleculars basats en ADN i de la morfologia. S'avalua el paper de la hibridació natural la variabilitat, l'origen i la conservació d'aquestes espècies endèmiques. / Natural hybridization is a widely known process in plants. It is an important source of variation promoting species evolution. It is likely to be the origin of many angiosperms, including local endemisms. Oppositely, it is also regarded as a potential threat for endemisms survivorship, diluting their differentail traits. This thesis deals with putative natural hybridization processes involving three Balearic endemics (Viola jaubertiana, Lotus fulgurans i Helichrysum crassifolium), from the points of view of the DNA molecular markers and the morphology. The role of natural hybridization in the variation, origin, and conservation of the above endemics is evaluated.

Comparative studies on endangered species and their more common congeners can shed light on the mechanism of species rarity, and enable conservationists to formulate effective management strategies. I compared the breeding and pollination systems of the endangered Geodorum eulophioides and its two more common sympatric congeners in subtropical China. Geodorum eulophioides and G. recurvum were self-compatible, both depending on Ceratina cognata for fruit production, while G. densiflorum can autonomously self. Although the floral visitation frequency of G. eulophioides was the highest among the three, its natural fruit set was similar to that of G. recurvum, but both lower than that of G. densiflorum. These results partially explain the difference in species abundance. Coupled with severe habitat loss and degradation, the extremely low pollinator visitation and natural fruit set of G. eulophioides calls for rapid establishment of ex-situ collection, in conjunction with improving in-situ habitat. Natural hybridization tendencies were also studied among species.

The thesis was focused on the microevolutionary mechanisms that contribute to morphological diversity in selected members of the sedge family (Cyperaceae). Natural hybridization, evidenced from both morphological characters and molecular markers, was revealed to be a potentially important source of diversification in the tropical spikerushes of Eleocharis subgenus Limnochloa. High levels of phenotypic plasticity of clonal growth but rare genetic (ecotypic) differentiation among contrasting morphotypes were found in the polymorphic species Carex nigra, which implied that taxonomic splitting of the species was unreasonable.