Primulas - pin and thrum

So what's the deal with primulas? As far as I know, Darwin first discovered they reproduced via heterostyly, whereby one individual with a prominent style requires a corresponding plant with a a shorter, "thrum", style. This prevents or reduces the chances of pin plants from producing more plants like themselves, and so on.

What are the benefits of this kind of reproduction? Why does it occur? Is it restricted to the Primulaceae family or related families? What happens if an insect passes pollen from a pin flower to another pin flower, or thrum to another thrum? Will you find pin and thrum flowers on the same plant? What are the effects of heterostyly? How can it be explained in simple terms, ie if explaining to children?
When a pin and thrum flower reproduce, what is the offspring? Pin or thrum?
Thanks.

 

Maybe I've put potential answerers off with asking too much above. To put it simply, what I'd like to know is what are the benefits of pin/thrum pollination ("why" it happens), what are the effects,and why it is apparently restricted to Primulaceae? Thanks.

 

So sorry Simon, I have never heard of "pin" and "thrum" !!

Val

 

As far as I am aware each individual plant only produces one kind of flower, either pin or thrum. Thus cross fertilisation is required for the plant to set seed. Cross fertilisation ensures that there is genetic variation within the species. Of course it also means that a plant is self infertile. Seed will produce roughly equal numbers of either pin or thrum eyed plants.
There are many plants which have male and female flowers on separate plants, again ensuring that genetic material is needed from tow individuals.

 

Well, basically if you look closely at the flowers of primroses -let's stick with P. vulgaris for convenience's sake - you'll see some have long styles sticking right out, others have centres with the styles much further embedded ("thrum") which can only be reached by long-tongued insects. Although its possible from like-styled plants, pollination is much more likely and produces healthier offspring via "heterostyly" which means a pin plant pollinating a thrum or vice versa. Darwin was the first to notice this and experiment, he had various theories as to why it occurred, all of which were disproved as he found the results. It is now generally considered to be a way of encouraging cross-pollination, but I can't understand why this is beneficial: what botanical advantage is accrued by having two different types of plants in the same species, and why does only a pin pollinate a thrum etc? Of course, I say "why" as if we're talking about people making decisions, whereas what I'm really getting at it what are the effects, and how is it of benefit in Evolution?

 

Thanks. But what advantages does this genetic variation cause? Why is it better for the species to have both pin and thrum? What variation does this cause?
Surely the fact it means individual plants are self-infertile is a disadvantage when it comes to ensuring higher numbers of plants, and thus species survival?

 

Genetic variation enables a species to "adapt" to a wide range of conditions at the extremes resulting in new species. Inbreeding (self fertilisation) results in the accumulation of genetic faults leading to weak plants, early death etc. As Fat Controller says like interbreeding dogs or marrying your cousin.
Pin and thrum are just another way of ensuring cross fertilisation.

 

Thanks. I suppose my confusion arises from the fact that not all plants employ this technique. For instance, a clump of snowdrops are growing right next to the primroses by my local canal, the snowdrops do not require this type of pollination yet thrive just as well as the primulas.
What are the advantages that a pin plant, for example, would have in growing in the same environment as where thrum plants also thrive? What would happen if they didn't ave this function? If only pin plants survived? What would be the problem?
And if I am a pin plant and my pollen fertilizes a thrum, what will the resultant plant(s) look like? Pin, or thrum??
Thanks!

 

Don't know anything about Pin and Thrum, and no idea if this is relevant, but just chucking it into the mix in case of interest:

Many Apple varieties are self-sterile, or partially self-fertile. Although the partially self-fertile may be fertilised by pollen from others of the same variety they tend to only set fruit biennially, or not very much, or not at all (particularly in the case of Triploid varieties). So there must be something about pollen from other varieties that makes it able to pollinate another variety (ANY other variety, except itself perhaps?)

Perhaps this is just the same as Pin and Thrum - a desire to cross-pollinate with only more distant relatives to provide greater genetic diversity, and thus greater possibility of surviving disease / drought etc./ because off spring will be more diverse.

Downside is that nature has to disperse some non-near-relatives CLOSE to you, whereas those species that are happy to mate with close relatives have the benefit that those relatives are all-around-them.