Abstract
ABSTRACT Waltheria encompasses about 60 species distributed in Brazil and Mexico. To improve the palynology of the genus, we analyzed 14 species occurring in the Brazilian State of Bahia. Pollen grains were acetolyzed and examined using light microscopy and scanning electron microscopy. Non-acetolyzed pollen grains were viewed by transmission electron microscopy. The results revealed two general pollen types based on exine ornamentation, such as reticulate and echinate, which are distinct with regard to (1) size and shape, (2) apertural type and number, (3) relative length of ectoapertures, and (4) thickening of the nexine compared to the sexine. An ultrastructural analysis of the pollen wall revealed a continuous foot layer and a compact and discontinuous endexine in most cases. The sexine consisted of heterogeneous columellae, which support a perforated semitectum (echinate type) or the semitectum was composed of perforations and muri-shaped thickening (reticulate type). These results confirm a high degree of pollen dimorphism in Waltheria associated with heterostyly.
Highlights
Waltheria (Malvaceae – Byttnerioideae) is a primarily neotropical genus, composed of mostly shrubs and subshrubs that bloom for several months during the year
The results revealed two general pollen types based on exine ornamentation, such as reticulate and echinate, which are distinct with regard to (1) size and shape, (2) apertural type and number, (3) relative length of ectoapertures, and (4) thickening of the nexine compared to the sexine
The sexine consisted of heterogeneous columellae, which support a perforated semitectum or the semitectum was composed of perforations and muri-shaped thickening
Summary
Waltheria (Malvaceae – Byttnerioideae) is a primarily neotropical genus, composed of mostly shrubs and subshrubs that bloom for several months during the year. It is distributed in two major centers of diversity, such as Brazil and Mexico, and encompasses about 60 species, of which 21 occur in Brazil, including 13 endemic species (Saunders 1993; Esteves 2015). Stamens are shorter than styles in longistylous morphs, whereas brevistylous morph styles are shorter than stamens This floral heteromorphism is genetically controlled, and the reciprocal length of the gynoecium and androecium in longistylous and brevistylous flowers favors cross-fertilization between them, considering that most species are self-incompatible (Ganders 1979)
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