A study on Self-Incompatibility of the genus Restrepia

[Tindomul]

Hi all,

This paper was just published this week in the American Journal of Botany. It basically tests the level of self-incompatibility in the genus Restrepia. Self-incompatibility is a mechanism found throughout the plant kingdom which reduces or stops a flower's ability of becoming fertilized by it's own pollen. The benefits are obvious in that it reduces genetic depression caused by inbreeding. It encourages out-breeding thereby increasing genetic variation in future generations.
There are many ways this occurs, sometimes plants produce pollen before the female parts are receptive, allowing pollen to wither before the ovary is ready to accept pollen. Sometimes the plants can recognize that the pollen on the stigmatic surface comes from the same flower (usually detection of certain proteins). So the plant inhibits pollen germination or it can inhibit the growth of the pollen tube down the pistil to the ovary.

Self-incompatibility is considered rare in orchids but is known to be present in Pluerothallids. This is strange since Pleuro's are the largest subtribe in the orchid family. There are 8 main clades in the Pleurothallidinae and 7 out of the 8 are known to have self-incompatibility (SI) mechanisms, and this paper confirms SI for the last group. All orchids also avoid self pollination with an array of pollination strategies that many of us here on this board are familiar with.

The paper gives some background info on the genus Restrepia. The center of diversity (area with largest concentration of species for this genus) is located in the high Andes of Colombia and Ecuador. They are also found in Central America, Venezuela and as far south as Bolivia. From my understanding of the paper, the author asserts that there are few cultivated hybrids between Restrepia species. Can anyone confirm this or elaborate on it? Most of the species in cultivation come from few original plants whom have been asexually reproduced (cloned or vegetatively propagated) to satiate the market.

The study was conducted on greenhouse Restrepia species. Flowers were hand pollinated where they tested the seed formation of flowers that were selfed, cross pollinated with other flowers of the same species and cross pollinated with different species.
The strength of the SI mechanisms were evaluated by the number of viable seeds that pollinations produced.

The results were that more seeds were produced after cross-pollinations (both interspecific and intraspecific). They noted an absence of pollen tube formations in selfings even after 9 days post pollination. They found that the minimum number of days for fertilization to occur was 9 Days. Even so, pollen tubes in selfings were not as extensive even 3 weeks later as compared to cross pollinations (see attached picture where image A shows no pollen tubes after 9 Days resulting from a selfing and image B shows extensive pollen tubes after 9 days resulting from cross-pollination).

Table 1 shows the percentages of viable seeds produced by each kind of pollination event. The first column showing low viability in selfings with a few exceptions at the bottom. The second and third column show high numbers of seedlings produced by cross pollinations (hybrids and interspecific alike).


The strange part is that some selfings worked, even if they resulted in low yields. The authors attribute this to the varying ability of the SI mechanisms (trait plasticity) and also because SI mechanisms weaken with increased age of the flower.
This study also support's C. Leur's proposal to separate R. aberrans and R. chocoensis into their own monotypic genera (Echmeles & Pachymeles respectively), given that their SI mechanisms worked so differently from the other species (see Table 1).

The authors go on to further suggest (and I agree with the plausibility of their logic) that because Restrepia's are indeed a genus with self-incompatibility mechanisms, that their fragmented habitats present an even higher risk to their species survival than previously thought. If they need cross-pollinations to maintain their genetic viability, they may not be able to successfully cross-pollinated if a suitable "mate" is located many miles away, further than their pollinators can travel. Their pollinators are supposed to be flys (myophily) by the way, but this remains unconfirmed in many species.

The authors also suggest that past studies of self-incompatibility in other orchids may not be as valid as previously thought because they have been based on the production of the seed capsule rather than seed viability. Seed capsules, they argue, can be produced just from the act of pollination even in the absence of fertilization. So they suggest that further studies should be made to check on the SI mechanisms of other orchid genera, old world and new world alike.

[WhiteRabbit]

Very interesting!