Chromosome Doubling Mechanisms
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There are two major chromosome doubling mechanisms which are involved in polyploid evolution: 1) somatic doubling, and 2) unreduced gamete formation. Somatic doubling refers to chromsome doubling in ‘somatic’ (or body) cells undergoing mitosis, while unreduced gamete formation occurs through meiosis. I must cover some basic nomenclature first, before I explain these mechanisms in more detail. “Ploidy” refers to the number of chromsome sets in an organism. A haploid organism has one complete set of chromsomes; a diploid has two; a triploid has three; etcetera. Humans are diploid organisms -- they have 2 sets of 23 chromosomes. Gametes (sperm and eggs) typically have a haploid number -- i.e. they have only one chromosome set. Most animals and fungi are diploid, however, many plants are polyploid (i.e. they have more than two sets of chromosomes. Apparently the chromosome number in plants ranges from 4 to over 3000 chromosomes in different species! In the examples below, I will assume that the parental plants are diploid. A genome is typically symbolized by an arbitrary capital letter -- different but closely related species (e.g. species with the same genus name) would each be given separate letters (e.g. A, B, C, D, etc). A diploid species may be symbolized by AA, and a related diploid species by BB. Triploids are then symbolised by AAA or BBB, and tetraploids by AAAA or BBBB. A hybrid plant may then be represented as AB, AAB, AABB, etc. Chromosomes from different species may be distinguished by a number of criteria. For example, the karyotype or morphology of chromosomes can be used to identify parental species of hybrids. Isozyme analysis (i.e. isozyme gel electrophoresis) may also be used for identification and analysis of hybrids. In my examples, circled letters represent either pollen grains or ovules. Uncircled letters simply represent an organism with a particular genome type. For example:
The letter ‘x’ is used to represent the ‘base number’ of chromosomes in a group of closely related species, and a number in front of it represents the ploidy level (i.e. the number of chromosome sets that are present). Thus, x would indicate a haploid organism, 2x a diploid organism, 3x a triploid, 4x a tetraploid, and so on. Sometimes the letter ‘n’ is also used to represent chromosome content -- e.g. n is haploid and 2n is diploid. To confuse things further, sometimes n is equal to x, but not always. I will attempt to be very clear when I use these symbols. For instance, I will use 2n to represent the somatic chromsome number of an organism, while n will be used as half of the somatic number. Somatic doubling (Fig-1A) occurs in cells which do not make gametes. Some failure in mitosis prevents normal cell division after the chromosomes have doubled. In plants, this can occur to yield an entire plant that has double the chromsome number of its parent, or it can occur in a single part of the plant (e.g. a stem or flower bud). For example, in a diploid plant, the somatic doubling of a bud can give rise to a tetraploid branch.
Crossing two different (but perhaps closely related) plant species will give rise to hybrid offspring -- offspring with two different genomes (Fig-1B). Diploid hybrids are typically sterile or ‘mostly sterile’. Tetraploid hybrids, on the other hand, are often fertile.
The mechanism for somatic doubling in nature is uncertain, but certain chemicals (e.g. colchicine) which inhibit spindle formation during cell division can be -- and have been -- used in the lab to create polyploid plants. Unreduced gamete formation is the second major chromosome doubling mechanism. Typically, gametes (e.g. pollen and ovules) are haploid. Actually, pollen is more correctly referred to as a ‘microgametophyte’ -- i.e. pollen is not a gamete itself, rather it is a structure that contains sperm (or microgametes). For simplicity, however, I will refer to pollen as a gamete. Gametes are formed by meiosis and are typically haploid (Fig-2A). Gametes may then be represented by n. Meiosis is a process which divides the chromosome number of the parent in half (fertilization then restores the original somatic number). Sometimes, however, failure in meiosis can produce what are known as ‘unreduced gametes’, or gametes with the somatic chromosome number (i.e. a diploid organism would produce diploid or 2n gametes) (Fig-2B). Unreduced gamete formation is considered to be rare, therefore a diploid plant may produce low frequencies of 2n gametes along with the normal n gametes. Plants generally produce much more pollen than they do of ovules, therefore 2n pollen is probably more likely than 2n ovule formation. In my examples, the unreduced gametes will refer to 2n pollen (rather than 2n ovules).
Autotetraploids (organisms with more than two genome sets -- or homologous chromosome sets -- from a single species) can arise through the mechanism shown in Fig-3. A diploid plant produces some frequency of 2n pollen. This 2n pollen then fertilizes a normal ovule, which produces a triploid plant. Triploid plants are typically sterile (and sometimes inviable), since there is an odd number of chromosomes. There are, however, a few plant species which are triploids (e.g. dandelions are triploids and produce seeds asexually by apomixis). Proper meiosis requires an equal number of homologous chromosomes for segregating normally to opposite poles in the cell. Sometimes, however, these triploid plants do produce viable unreduced pollen (i.e. triploid pollen) which can then fertilize a normal haploid ovule. Ultimately, this produces tetraploid offspring as demonstrated in Fig-3.
A similar process works to produce allotetraploids (organisms with more than two genome sets from different species). This is shown in Fig-4. A diploid plant from a species with the A genome produces unreduced pollen which fertilizes an ovule from a second diploid species with the B genome. This produces a diploid hybrid with the genome constitution AB. This allotriploid then produces unreduced pollen which fertilizes an ovule from the B species. This generates an allotetraploid with the genome constitution AABB.
There are many variations in the above processes which give can rise to different polyploids. For example, once a tetraploid has arisen, similar mechanisms can work to generate hexaploids and octoploids. Add a third species (e.g. with genome C or D) and a variety of allotetraploids may result. One of the world’s most important food crop, wheat, is hexaploid (AABBDD) with genomes from three different, but related, species. I have written an essay on polyploid evolution, which goes into more detail on how polyploids evolve. In particular, I focus on three important areas: 1) a frequency-dependent process (referred to as the ‘minority cytotype exclusion principle’), 2) factors that influence unreduced gamete formation, and 3) genetic diversity and multiple origins of polyploids.
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