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QUESTIONS AND ANSWERS
Plasticity in the
self-incompatibility system of Solanum carolinense and its impact on
the molecular evolution and ecological genetics of S-alleles. Andy Stephenson, Pennsylvania State Univ.
Q: Does S. carolinense hybridize with any
other species?A: To my knowledge
it does not hybridize with any sympatric species of Solanum (most of the
weedy/viny Solanum's are auto- or allotetraploids such as S. nigrum
and S. dulcamara but S. carolinense is a diploid...).
Q: Are mechanisms for seed and pollen dispersal known for horse
nettle? (My own experiments show that rabbits disperse seeds in their feces.)
A: I do not know how the seeds disperse. They
seem to linger on the dead stems for a long time (into the winter in northern
regions). I suspect that field rodents also take the dried fruits and eat and/or
cache the seeds. I've never seen a bird eat the fruit of S. carolinense.
Rabbit dispersal is interesting!Q: Is
there a senescence signal transduction cascade initiated besides RNAse activity
inside the pollen tube to stop pollen tube growth? If a senescence cascade is
initiated, are the contents of the incompatible pollen tube catabolized and
distributed to the compatible pollen tubes for growth?
A: Neither I nor anyone else to my knowledge
knows exactly what happens after the RNase enters self pollen tubes. In vitro,
the RNase will breakdown mRNA and rRNA which would definitely retard the rapidly
growing pollen tubes. rRNA is very important for vesicle formation and 3000-8000
vesicles fuse with the tip of growing pollen tube per minute. These vesicles not
only add membrane to the growing tip of the pollen tube but they also release
enzymes into the transmitting tissue of the style that are believed to play a
role in the metabolism and uptake of resources from the maternal tissue. In
addition to fusing with the tip of the pollen tube, some vesicles blip off of
the tip, flow away from the tip in the cytoplasmic stream, and transmit maternal
resources to areas behind the tip of the pollen tube (containing mitochondria
and the vegetative nucleus). Although the precise sequence and cascade of events
following the RNase entry into the pollen tube is unknown...it is possible to
watch what happens to self pollen tubes. The growth of self tubes slows down and
then the tip of the pollen tube blows up like a balloon and bursts. It is not
known if the contents of the ruptured pollen tubes provide nutrition for the
healthy tubes but I would doubt it because the healthy tubes (non-self tubes)
have already passed by the self tubes when they slowed down. Only the tip of the
pollen tubes is alive (callose plugs isolate the tip from the older part of the
tube) so the "old pipe" is not able to take in resources and use them (I don't
think).
Q: What specific effects do temperature and humidity have on the
strength of self-incompatibility?A:
We haven't done any experiments on the impact of temperature on the strength of
SI in horsenettle...however, warmer/hot temperatures are known to make SI
"leaky" in several species with various types of SI (heteromorphic SI,
Sporophytic SI, other types of gametophytic SI and the Solanaceous type of GSI).
Consequently, it would not surprise me to find out that temperature affected the
strength of SI in horsenettle (probably by accelerating the rate of S-RNase
turnover in the styles).
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