Comparative Morphology of Chloroplasts in Podostemaceae Subfamilies Tristichoideae and Weddellinoideae suggests Evolution of Chloroplast Dimorphism

Podostemaceae comprise a unique aquatic angiosperm family. Members of the family grow on rock surfaces of waterfalls and rapids in the tropics and subtropics. Recently, chloroplast dimorphism was reported for 13 species from the majority of clades in the subfamily Podostemoideae. Large chloroplasts with well-developed starch grains and small chloroplasts with few starch grains are located separately in the epidermal cells of roots and shoots. Transmission electron microscopy (TEM) analysis revealed that the large chloroplast is comparable to a typical, ordinary chloroplast, While the small chloroplast does not, but is specialized. To investigate whether chloroplast dimorphism is common in Podostemaceae, we conducted TEM and light microscopy of six species from two subfamilies, Tristichoideae and Weddellinoideae. All samples examined had uniform chloroplasts of the same size. Evaluation of their ultrastructure indicated they had normal grana and starch grains. These findings suggest that chloroplast dimorphism is a trait limited to Podostemaceae subfamily Podostemoideae.