Ever wondered why apple trees are mainly self infertile although they have male and female parts in each blossom?

Apple trees belong to the largest group of plants by far - angiosperms. Angiosperm  translates to “vessel seed” which refers to ovules being enclosed in the female gynoecium or carpel. The carpel is one of the organs that make up what is arguably the defining feature of the angiosperms, the flower. While flowers in extant species display an astonishing diversity of sizes, shapes, colours, and functions, a central feature of their ancestral state, hermaphroditism, has remained widespread, and is present in apples, and brings the  pollen‐producing anthers in close physical proximity with the entry of female reproductive tracts, the stigma.

It follows that this structural arrangement  can cause self‐pollination to be very common, such that the potential for sexual reproduction by selfing is high. To avoid the deleterious effects of inbreeding depression, reproductive mechanisms preventing selfing have been repeatedly selected in the course of evolution. The basic flower has evolved to attract insects for pollination and consist of petals surrounding both male and female sexual organs, the male stamens outside the central female stigma. Physically this evolved design encourages self fertilisation when the bee or other insect  transfers pollen to the stigma.

However some plants have moved away from that, apples being one. Evolution in changing times preferring the random shuffling of genetic material between different members of the same species,  generating and maintaining genetic diversity. Plants that reproduce clonally  at the other end of a spectrum show little or no variability making them vulnerable to climatic changes. Thus, although many plants reproduce by selfing, many others including apples have evolved special mechanisms to enhance outcrossing. This is called Self Incompatibility (SI) , and therefore requires mechanisms to prevent a successfully fusion of gametes from the same flower. Several different  genetically controlled mechanisms have evolved. In apples the mechanism used involves recognition of and 2 way interaction between the self pollen and the pistil ( the female parts)  and then destruction of the pollen tube (which grows out of the pollen grain down the style (which connects the stigma via the style to the ovules in the ovary) and contains the sperm genes and is genetically  male). It is the expression of  S-RNase in the style, and  its interaction with S-locus F-box brothers (SFBB) in the pollen tube membrane which  arrests pollen tube growth when both are ‘self”. 

A little explanation here. The S locus in apples  is complex of genes that encodes separate proteins for style and pollen -extracellularS-ribonucleases or S-RNases for short, in the style; and F-box proteins in the pollen tube respectively. Ribonuclease  is a type of enzyme that catalyses the degradation of RNA into smaller components.) S-RNase is secreted from the style, enters into the pollen tube. If the S-RNase is  not recognised as ‘self’ by the pollen’s SFBB, it is labeled with ubiquitin which leads to the S-RNase destruction and therefore no effect on pollen tube growth. If the   S-RNase is recognised as self it is not ubiquitinated and remains in the cytoplasm of the pollen tube and has  a cytotoxic effect on the tube halting its growth so the pollen DNA in the sperm organelles cannot get to the ovule.

( as an aside - I love the name - ubiquitinated! Ubiquitin is a small regulatory protein found in most tissues of eukaryotic organisms, i.e., it is found ubiquitously! It was discovered in 1975. Ubiquitination is a tightly regulated, highly specific, and ATP-dependent biological process carried out by a complex cascade of enzymes. Ubiquitination is an essential player in protein homeostasis, serving to rapidly remove unwanted or damaged proteins

Cultivars with the same S-RNAse and SFBB alleles therefore effectively cannot fertilise each other. Fruit set in most apple cultivars is less than 10% after self-pollination from this mechanism.

So a complex to and fro. Finally decided by whether the pollen recognises the RNase of the  female part it is growing into. If it  recognises the female RNase is not destroyed, but  the pollen tube is destroyed. Take care not to recognised yourself!