Wild roses, like this sweet briar (Rosa rubiginosa) in Canterbury, don't look much like the cultivated roses in most people's gardens. Why not?
A flower is a specialized branch, with four kinds of leaves: sepals, petals, stamens, and carpels. How they develop is governed by a set of floral homeotic genes. If one of these genes is non-functional due to a mutation, some of the kinds of floral leaves don't develop properly, or they develop into the wrong kind of leaf. Many cultivated roses have their stamens converted into petals, so instead of five petals like the flower above, they have lots, and they don't produce pollen. Plants with flowers like those, called double flowers, tend not to reproduce, so the mutant form of the floral homeotic gene tends not to survive in wild populations, but it is successful in cultivation because the extra petals add to the showiness of the flowers. Gardeners select these sterile mutants and propagate them. Carnations are a similar cultivated double form derived from wild pinks.
The rose family has a huge diversity of fruits. Think rosehips, apples, strawberries, raspberries, plums and cherries, and the dry fruits of Spiraea and Acaena. These fruits are all made of the same building blocks, but they differ in which parts are fleshy, how many carpels are in each flower, whether they're joined or separate, and whether the carpels open to release the seeds or not.
England should comfortably beat Georgia tomorrow, but then they face Scotland. Again, history is important: Prestonpans, Culloden. Why is it that geographic proximity seems to cause the most intense rivalry?
Fascinating about the double flowers! I never understood that properly before. So how does that work - the conversion of stamens into petals?
ReplyDeleteHomeotic genes control how bodies develop; they're really important in our own bodies, determining where different organs are produced. The same genes control similar development in insects and mammals, which is part of the raft of evidence that living things are related through common descent. For instance, there's a fruit-fly gene called EYELESS that turns on the development of eyes (some genes are confusingly named according to what happens when they fail to work). If you take an eyeless fruit-fly embryo and replace its defective EYELESS gene with a functioning eyeless gene from a mouse, it'll develop eyes again: not mouse eyes, but proper fruit-fly compound eyes. That shows the gene instructs other genes to build an eye, but it doesn't control how the eye is built. Our bodies are segmented too (think ribs, vertebrae) and the homeotic genes control how each segment differentiates.
ReplyDeleteIn plants it's a bit different, and flowers develop according to three classes of homeotic genes. Class A genes control the outer two whorls of floral leaves (sepals & petals), class B the 2nd and 3rd whorls (petals and stamens), and class C the innermost two whorls (stamens and carpels). It's the AB combination that determines petals, and the BC combination determines stamens. If class C is missing, the developing 3rd whorl is determined by the AB combination to develop as extra petals instead of stamens (which require BC); often the innermost whorl develops as sepals (A) instead of carpels (C), but I'm not sure what happens in roses. There's also an overall master-switch called LEAFY, which turns a developing branch into a flower (when it doesn't work, a stunted leafy shoot replaces the flower).
See http://en.wikipedia.org/wiki/ABC_model_of_flower_development
Here's a link to Jerry Coyne's website, where he describes an animal equivalent.
ReplyDeletehttp://whyevolutionistrue.wordpress.com/2011/09/28/a-twelve-year-old-cat-with-two-faces/
"Sonic hedgehog" is an animal homeotic gene (yes, sometimes genes are named in even sillier ways than naming them after what happens when they don't work).