Palynological study of heterostylous species of Melochia L. (Byttinerioideae-Malvaceae) occurring in Bahia, Brazil

Melochia L. species are morphologically characterized by a pentacarpelar gynoecium with free papillose styles at the apex, whereas some are connate. The genus comprises about 60 species, with 22 occurring in Brazil. We analyzed pollen grains from 12 heterostylous species occurring in the State of Bahia to contribute to the palynology of the Melochia Group, including Melochia betonicifolia A. St.-Hil., M. caracasana Jacq., M. chamaedrys A. St.-Hil., M. hirsuta Cav., M. illicoides K. Schum., M. parvifolia Kunth, M. pilosa (Mill.) Fawc. & Rendle, M. pyramidata L., M. spicata (L.) Fryxell, M. splendens A. St.-Hil. & Naudin, M. tomentosa L., and M. villosa Fawc. & Rendle. Pollen grains were provided from specimens deposited at the CEPEC, HUEFS, and SP herbaria. The floral morphs of all specimens had been previously studied with a stereomicroscope. Acetolyzed pollen grains were investigated by light microscopy, and non-acetolyzed pollen grains were observed by scanning and transmission electron microscopy. It was confirmed in two pollen types of the two floral morphs: (1) the longistylous morph was characterized as medium sized; prolate spheroidal; 3-colporate, with ectoapertures comparatively larger, endoaperture lalongate; reticulate-perforate; exine ultrastructure with nexine differentiated into endexine and a foot layer (M. pyramidata) or undifferentiated (M. betonicifolia); a 4-stratified sexine may also occur; (2) the brevistylous morph showed medium to large-sized pollen grains; oblate spheroidal; 3(4)-colporate, ectoaperture short, endoaperture lalongate; microechinate and echinate; semitectum perforate and microreticulate-rugulate (M. betonicifolia); M. pyramidata wall ultrastructure with nexine showing “internal foramina.” Pollen dimorphism was confirmed in the genus due to heterostyly. The type and number of apertures were not as useful as exine ornamentation to characterize and distinguish the pollen grains.