Studies On Pollen Structure And Evolution Of Chinese Boraginaceae

To study the taxonomy of the family Boraginaceae furtherly, the pollen morphology and exine ultrastructure of 61 Chinese species representing 20 genera belonging to the Cordioideae, Ehretioideae, Heliotropioideae , Boraginoideae, were examined by LM. , SEM. and TEM..Cordioideae: Four aperture types of the genus Cordia L in the Cordioideae were found. These types are 3-porate, 3-colporate, 3-syncolpate and 3-colporoidate. There are four ornamentation types on the exine surface: microspinulate, spinulate, reticulate and irregularly striatereticulate. The characteristics of pollen morphology in this genus show that Ccrdia L. is a unique taxon and a rather primitive genus in the Boraginaceae.Ehretioideae: Pollen morphology in ten species of Ehretia and each one species in Rotula and Carmona of the Ehretioideae, was observed. Pollen grains are prolate or subspheroidal, rarely spheroidal, more or less enlarged in the equatorial region, elliptic to subrhombic in equatorial view, and hexalobate-circular or obuse-trianular in polar view; the apertures are 3-colporate alternated with 3 pseudocolpoids, and rarely 3-porate; the exine is faintly microperforate, rugulate or psilate. The results show that the subfamily Ehretioideae is distinctly different from the subfamily Cordioideae in pollen morphohogical features, such as the pollen shape, the aperture number and type, and hence support the treatment of them as two independent subfamilies. Some pollen morphological features, such as the rather large size of poller grains and the equatorial region not being constracted, suggest that both Ehretioideae and Cordioideae might be primitive groups in the Boraginaceae, although the pollen grains in Ehretioideae seem to be of a higher evolutionary level tha n those in Cordioideae as shown by their shape and aperture features, and tnus indicate that the former subfamily might be more advanced than the latter. Pollen grains in the genus Rotula in Ehretioideae are 3-porate, implying the possible close relationship between Ehretioideae and Cordioideae.Heliotropioideae: Pollen morphology in six species of the Heliotropium , two species in the Messerschmidia and one species in the Tournefortia of the Heliotropioideae was observed . Pollen grains in these species are prolate, more or less enlarged in the equatorial region, elliptic in equatorial view, and 6-lobate circular, 8-lobate circular and rarely triangular' in polar view. There are three apertural types in the subfamily Heliotropioideae, namely: (1) 3-colporate type, (2) 3-colporatealternated with 3-pseudocolpate type, (3) 4-colporate alternated with 4-pseudcolpate type. Exine surface is faintly microperforate, rugulate ornamentation or rarely psilate. The results show that relationship among three genera in the subfamily Heliotropioideae of the Boraginaceae is close. Based on pollen features the genus Tournefortia might be more primitive group in the Heliotropioideae of the Borginaceae, the Heliotropium seem to be of a higher evolutionary level than both the Tournefortia and Messerschmidia, and the genus Messerschmidia lies between the genera Heliotropium and Tournefortia. Pollen morphological features exhibit that the subfamily Heliotropioideae is a transitional type from Ehretioideae to Boraginoideae; and pollen grains in the genus Tournefortia of the Heliotropioideae are 3-colporate apertures, implying the possible close relationship between Heliotropioideae and Cordioideae.Boraginoideae: Pollen morphology of 16 Chinese species representing 7 genera in the tribe Cynoglosseae of Boraginoideae was examined under LM. and SEM. , and 5 of them under TEM.. Pollen grains are cocoon-shaped, rarely subprolate, prolate or ovoid, very small, 7 ~ 15. 7X3. 5 ~ 13. 9Mm in size, P/E=l. 6 ~ 2.02; 3-colporate apertures alternate with 3-pseudocolpi, with equatorial endocingulus except these in Bothriospermum; exine surface usually smooth, with or without perforations in two poles, rarely with tubercuiate ornamentation; exine is rather thin and includes ectexine and endexine, while ectexine consists of imperforate tectum, columellae and foot-layer. As mentioned above, the tribe Cynoglosseae has many common characters of pollen grains, but there are some differences among genera, a key is provided. Bothriospermum without endocingulus is a primitive genus in this tribe, and the pollen morphology is more similar to Eritrichieae than to Cynoglosseae, thus it is reasonable to put it into Eritrichieae. Solenanthus is the most advanced one because of its subisopolar and ovoid shape.Pollen morphology of 15 species representing 5 genera in Trib. lithospermeae of Boraginoideae was examined under LM. and SEM.. Pollen morphology of the tribe is distinctly eurypalynous. Pollen, grains are subspheroidal, prolate, cocoon-shaped, dumbbell-shaped and oviod;10.4 ~ 41. 8X7 ~ 33. lMm in size;there are five types of aperture, 3-colporate,3~ synco]porate, 4 ~ Scolporate, 4 ~ 6syncolpate and 6 ~ 7eolpate type; exine ornamentation is generally smooth or rugulose, sparcely echinulate and rarely rugulate. A key tc the genera of this tribe vased on pollen morphology and a table of comparison of apertures of tribes in Boraginoideae are given. The evolutionary trends of pollen grains of this tribe areexplained. Finally, the taxcnomic position of this tribe in Boraginoideae and the taxonomic problem of Echium are also discussed; according to palynological data, the Trib. Lithospermeae may be the primitive position Boraginoideae for its eurypalynous type, and it seems more reasonable to put Echium in Trib. Lithospermeae than to establish a new tribe.