Pomes. What is in a word?
Pomes what does it mean? Apples are classed as Pome fruit. The word pome entered English in the late 14th century, and referred to an apple or an apple-shaped object. It derived from the Old French word for the apple pome 12th century. (Modern French has now 2 ‘m’s as pomme). The Old French word derived from the early Latin word poma from where the Goddess’ name Pomona derives from to.
In botany, a pome is a type of fruit produced by flowering plants of the tribe Malineae of the very large family Rosaceae
Pomes all share certain structural similarities. Pomes are accessory fruits, ie their edible flesh forms not only from the ovary but from other parts of the flower. The fused carpels (the female reproductive organs of the flower) encase the seeds to form each pome's core. The endocarp (a tough, fibrous flesh) surrounds the seeds, and the softer mesocarp (the pome's edible flesh) encases the endocarp. And finally the pome's epicarp (the skin) protects the edible flesh.
But there is only one group who form false fruits like this. And they have a curious chromosomal count of 17 rather than 7-9 chromosomes that the the rest of Rosaceae have. And these 17 chromosomes are derived from the original 7 with much duplication of some whole chromosomes, parts of chromosomes, and some deleted and moved segments of the chromosomes.
By tracing genome alterations over time, this Genome Wide Duplication event has been dated to around 60 +/- 10 million years ago. It occurred only in this tribe, which is spread around the world; in apples pears medlars hawthorn and loquats to name the most common.
Expansion of the gene segments responsible for fruit formation gives the pome a false fruit caused mainly by the expansion of the receptacle and lower septal region.
In apples a sub clade of genes called MADS-box genes is expanded to 15 and similarly are the genes for metabolism and sorbitol transport. So the apple that resulted has a large fruit filled with sugars. An ecological niche that attracts large mammals such as bears and deer. (In plants, MADS-box genes are involved in controlling all major aspects of development, including male and female gametophyte development, embryo and seed development, as well as root, flower and fruit development. MADS-box genes are detected in nearly all eukaryotes studied, both plant and animal.)
Different apple characteristics such as size colour acidity flavours are controlled by different number of duplication of these paralogous copies, hence the wide range of apple colours and taste.
This Genome Wide Duplication event occurred before the separation of species into apples and pears and occurred at the same time as did some other plant species such as Poplar trees. There is little research into this event onto apple development but we are fortunate the apple and its genome has been extensively studied for apple storage and breeding of new cultivars for human consumption so the progression in the wild apple Malus sieversii be followed .
This rapid ( in evolutionary terms) evolution of new species of the Malineae trees is an example of adaptive radiation which occurs when new niches open up. So what caused this?
About 60 million years ago there was a major event that made the dinaosaurs extinct. An asteroid 10km in size impacted in the shallow sea near Mexico at Chicolub. ( now geochronologically dated by argon/argon method to 66,038000 years +/_10,000!) And from analysis of the rocks for pollewn it occured in the Spring.
However the asteriod impact did much more - 75% of all species went extinct, both plants and animals in the sea and on land. Such a change has been given a name the Cretaceous-Paleogene (K-Pg) boundary and is seen as a geological feature, a thin band of rock containing much more iridium than other bands around the world. Asteroids contain much more iridium than the earths crust normally does.
Immediately after impact, the entire Earth was bathed with intense infrared radiation from ballistically reentering ejecta. The global heat pulse would have killed unsheltered animals directly, and ignited widespread forest fires. Carcinogenic compounds would have been formed by the burning forests. Sulphates from the rocks at the impact site would have given rise to carcinogenic sulphur compounds. This period of 2 weeks was then followed by 2 years of cold and lack of sunlight, due to the dust and soot in the upper atmosphere. The temperature dropped 10C. Most plants on land and phytoplankton in the sea would have stopped photosynthesising from lack of sunlight. The ozone layer would have been destroyed allowing mutagenic levels of cosmic radiation to reach the Earth’s surface.
This phase was then followed by a period of over warming as heat became trapped by the CO2 release after the atmosphere cleared of dust.
An extremely hostile environment but ideal for adaptive radiation of those new species that formed. Reduction in competitors and a harsher environment including mutagenic effects. During this period of rapid change it is thought that evolution was more of a web of changes than linear as usually depicted by a tree branching pattern with species intermingling until enough separation genetically so as not reproduce together. Many of these new mutations of course would not survive as there was no niche for them. For the apple this was not the case. Small and then larger mammals replaced the dominant retiles of the previous Cretaceous period. Fossil evidence shows that mammals rapidly increased in size over the 10 million years post K-Pg event. They relied on smell to forage. Increased brain size and intelligence came later.
So an asteroid impact was bad news for the dinosaurs but good news for the apple and humans.