Ploidy levels of roses?

Hey Rip... school biology is about all you'll need to start to get a handle on this ploidy matter in roses...

Ok... First thing you need to know, or refresh your memory of, is that in living things like you and me there are two copies of every gene; one from Mum and one from Dad. That also means that every chromosome has a match (again; one with genes from Mum and one with genes form Dad). Because we have two copies of every gene that makes us diploid (di=two). In some organisms, like plants, it doesn't always work like this. There are diploid roses whose genes are paired like that of yours and mine, but there is also quite a lot of variation in between.

When sperm and eggs are made in animals the genetic material must first be divided in half. That is, each gene pair must be separated and one goes one way and the other goes the other way. This happens in both eggs and sperm and at the most basic level of understanding it just means that each one of the paired chromosomes gets pulled apart and one goes into one sperm/egg and the other goes into another. This is why we call sperm and eggs in animals haploid. This means half the number of chromosomes. When the sperm and egg get together in fertilisation the two lots of chromosomes mix again resulting in a full set of chromosomes again (i.e. a diploid individual again).

In roses when the chromosomes are meant to separate to form haploid pollen or eggs (ovules) sometimes they don't and all the chromosomes can end up in a single pollen or a single ovule. In these cases the pollen or ovules are not called haploid because the chromosome number has not halved. It is diploid. Now certain groups of roses tend to show tendencies to make diploid pollen and ovules. Hybrid Tea, for example, tend to make diploid pollen. This means that each pollen grain and each ovule contains 2 sets of each chromosomes and this means that when diploid pollen fertilises diploid ovules you end up with FOUR copies of each chromosome. We call these tetraploids because tetra means four. Now... so what you say... the thing we often see as a result of this is that all the flower parts are bigger, the pollen has a larger diametre, the ovules are larger (and when you thnk about it this makes perfect sense because there is twice as many chromosomes in each cell as in a diploid so the cells need to be bigger to hold it all). There are other changes too. It also makes inheritance a pain in the bum to try and work out or predict. In humans, for example, we have one gene for blue eyes and another for brown eyes. It is possible for an individual to have one of each because we get one from Mum and one from Dad. If we do have one of each we still have brown eyes though because brown masks blue (or in other words brown is dominant and blue is recessive). If we have two brown-eye genes our eyes will be brown too and if we have two blue-eye genes our eyes will be blue. If a brown-eyed person and a blue-eyed person have children together and one of their children has blue eyes I know the brown-eyed parent has a blue-eye gene that is being masked. If two blue-eyed people have a brown-eyed child... I know the bloke isn't the child's Dad it's that cut and dry (for this gene). In roses, if you have four copies of each gene instead of two, working out dominant and recessive traits is much harder, especially when there are more than two variations of each gene (as in the eye colour example). To make matters worse, there are a lot of genes, in both roses and people, that don't follow normal dominant and recessive patterns. To make matters even worse still (if that's at all possible I hear you say), if a rose is tetraploid and has four different versions of the same gene and one is more dominant to all the others all three of the other genes in the same place on the other chromosomes will be masked and you won't know they are there until you breed with them making the outcome very difficult to predict indeed!

In terms of breeding, ploidy is useful to know because it can affect how likely a cross is to take, the fertility of the resulting seedling, and features such as the physical size of the plant's parts, etc. You may have heard people talking about triploids? These roses are possessed of three copies of each chromosome (i.e. tri=three). This can happen when a diploid rose is crossed with a tetraploid rose. A diploid rose's pollen or ovules will be haploid (one copy of each chromosome) and a tetraploid rose's pollen or ovules will be diploid (two copies of each chromosome). Put them together and you have three copies of each chromosome making it triploid. Unfortunately many triploids experience greatly reduced fertility. This is because when the time comes to make pollen or ovules and the chromosome number needs to be halved an uneven number of chromosomes can make this difficult... you can't have one-and-a-half copies of each chromosome! There are many examples of fertile triploids, however, and one should not be discouraged from experimenting with them based on ploidy alone (EDIT: as someone for whom I have an enormous amount of respect for once told me, his motto is 'challenge you assumptions'. This applies as equally to roses as it does to everything in life. Don lives his life by it and Mr Moore also did... if it's good enough for them.. it's good enough for me too END EDIT). There are many other contributing factors affecting fertility such as environment and physical, or structural, barriers so I reckon it's worth a shot especially if there are already descendants listed on places like HMF. When working with species roses (except those like eglanteria, canina, and rubiginosa which show different meiosis again because they are pentaploid), we find they are very often dilpoid so when crossing back to modern tetraploid roses we mostly get triploids. This is why a lot of these roses form evolutionary dead-ends. You have to start somewhere though, and sometimes you come across triploids that have excellent fertility and so provide a way forward. EDIT: You are just as likely to find tetraploid x tetraploid roses that are infertile as well. My very first seedling, germinated 7 years ago now, affectionately known as 'Ugly Duckling' is a case in point. It was a self-pollinated hip from 'Black Velvet' and it produces no pollen at all and is almost certainly a tetraploid. END EDIT.

These are generalisations and there are other exceptions to the rule but I hope this has helped you understand the basics of ploidy and some of the effect it has on breeding roses.