Studies of the SI of the subtribe have focused on
Dendrobium.
Dendrobium is the second largest genus in
Orchidaceae (second only to
Bulophyllum)
[12], with approximately 1450 species. It is a perennial epiphytic herb that is mainly distributed in tropical, subtropical, and temperate regions of Oceania and Asia
[13].
By conducting more than 1700 pollination experiments on 61 species of
Dendrobium [3], Johansen found that 44/61 (72%) of the species had wilted and yellowed ovaries and showed self-sterility after self-pollination. Among the remaining 17 fruit-producing species, the fruit ripening time and seed quality varied considerably. Analysis of the development of the self-pollenated tubes of
D. fameri (SI) suggested that it was consistent with the GSI phenotype given that the pollen tubes enter the style. This study indicated that some species of
Dendrobium showed the GSI phenotype. Niu et al.
[14] studied 26 representative species of
Dendrobium and analyzed the pollen tube growth of 13 species. Four kinds of pollen tube growth phenotypes were observed in self-incompatible species: (1) pollinia did not germinate; (2) the pollen tube stopped growing at the top of the style; (3) the pollen tube stopped growing at a specific position in the stylar channel, mostly at the upper third of the stylar channel; and (4) the pollen tube stopped growing in the upper third of the ovary. For example, the pollen tube of
D. densiflorum stopped growing just before or after the style entrance one day after self-pollination. The self-pollinated pollen tube of
D. chrysanthum stopped growing at the upper third of the style three days after self-pollination. The pollen tube of
D. lindleyi stopped growing at the point before or just after the ovary entrance
[14]. The growth of the pollen tube in more than half of the self-incompatible species stops in the style, and these species are distributed in different branches of
Dendrobium phylogenetic tree
[14][15], which is the main SI phenotype and consistent with the GSI phenotype. The diverse pollen germination and pollen tube growth phenotypes suggest that there might be more than one molecular mechanism of SI in
Dendrobium species.
All these results show significant differences to those of molecular mechanisms known in other angiosperm families. There is a surprisingly high SI phenotype diversity in orchids, even in one genus, while there is only one SI phenotype described in other angiosperm families with their molecular mechanisms known, respectively. Furthermore, the emergence times of the SI phenotypes after self-pollination varies from species to species, mostly at three to five days, but even at two to three weeks, which much longer than that (mostly from tens of minutes to hours
[16][17]) in other angiosperm families with their molecular mechanisms known. Therefore, investigation of more other orchids SI species is needed, which may reveal more SI phenotypes.