BIOLOGY 101 – MITOSIS AND MEIOSIS – CHROMOSOMES
Chromosome numbers in Pelargonium(below)
BY WAYNE L. HANDLOS, Ph.D.
Among the higher plants and animals, most of the genetic material or DNA is found in discrete structures called chromosomes. The chromosomes in turn are normally found inside the cell in a spherical structure called the nucleus. Most of us “higher” organisms have two sets of chromosomes – one set received from our mother and one set from our father. These two sets came together through the process of fertilization when an egg and a sperm united. One set of chromosomes is referred to as the haploid or n or 1n number of chromosomes. In humans we receive a set of 23 chromosomes from our mother and 23 chromosomes from our father. Therefore in each of our body cells we carry 46 chromosomes. This chromosome number is referred to as the diploid number of chromosomes or the 2n number of chromosomes. In humans the chromosomes are numbered from 1 to 22 plus X and Y. A female has two each of numbers 1 to 22 plus two X chromosomes. A male has two each of 1 to 22 plus one X and one Y chromosome. After an egg is fertilized it is called a zygote. This cell then undergoes repeated nuclear and cellular division (this process is called mitosis) to produce an embryo and finally a complete human being.
In flower plants including geraniums, the process of reproduction is essentially the same. A haploid egg nucleus unites with a haploid sperm nucleus. The sperm nucleus is found in the pollen grain. When a pollen grain lands on a stigma (part of the female structure in a flower), a tube grows from the side of the pollen grain through the stigma, down the style into the ovary and ultimately into an ovule (the potential seed). Inside the ovule is an egg. A sperm nucleus moves through the pollen tube and ultimately fuses with the egg nucleus to form a zygote. The sperm carries one set of the anther’s chromosomes – the haploid or n number of chromosomes.
[I have simplified this process. Other nuclei are present and their origin and fate will be considered in another article.]
The egg carries one set of chromosomes from the female part of a flower. The union of these two nuclei (egg/ ♀ and sperm/♂) restores the diploid chromosome number in the resulting zygote. Subsequent cell and nuclear divisions produce an embryo – a small, baby plant normally found in a larger structure called a seed. [A number of parallel events may also occur in seed production but I’ll ignore those here.]
Now, how do we go from the diploid, 2n number, to the haploid number? There is a specialized process called meiosis in which the chromosome number is reduced by half so that the chromosome number remains the same from generation to generation. This specialized process is also important to sort out all the chromosome and genetic information from the two parents. While our genetic information is carried on DNA in chromosomes, each chromosome carries different information. So when meiosis is complete in humans our egg cells carry one each of chromosomes 1 to 22 plus one X chromosome (a total of 23) and each sperm cell carries either one each of chromosomes 1to 22 plus one X or one each of numbers 1 to 22 plus one Y chromosome. This special process, meiosis, achieves an orderly separation of chromosomes by first having all the chromosomes pairing up with their respective partners. The chromosomes only pair up with their exact partner, # 1 from the female with #1 from the male, #2 from the female with #2 from the male, #3 with #3, etc. and X with Y, although in humans the X chromosome is much longer than the Y chromosome. Normally the orientation of the paired chromosomes is random so at separation, egg cells and sperm cells carry a random assortment of the parental chromosomes though there will be a complete set of chromosomes 1 to 22 plus X or Y. Human chromosomes 2n = 46 >
Meiosis occurs somewhere in the life cycle of all sexually reproducing plants and animals. Occasionally mistakes occur in this process. In humans an extra chromosome may get included in a reproductive cell. This can lead to a zygote with a diploid number of 47. People with Down’s syndrome have an extra chromosome. The developmental process is disrupted to some degree by the additional chromosome. The addition of an extra chromosome of other sets of chromosomes may lead to other syndromes or the resulting zygote/embryo may be unviable or be spontaneously aborted. There are many plants that show a departure from the expected chromosome number. To get back to geraniums, many studies have been done to count the chromosomes of our favorite plants. The following examples show some of the range in numbers.
However, it is sometimes possible to create hybrids between plants with different chromosome numbers. Occasionally a diploid individual or a branch of a plant will have a doubled chromosome number. This plant is a tetraploid (4n). Horticulturists frequently find such sports desirable. Flowers in these plants are often larger, the leaves are thicker, the overall plant is larger, yield may be higher, etc. When this occurs the diploid (2n = 22, n = 11) crossed with a tetraploid (2n = 44, n = 22) is likely to yield a triploid (2n = 33). Such plants are usually sterile. One of the reasons for this can be seen in the process of meiosis. When the chromosomes come to pair, there are three sets of each chromosome. When meiosis proceeds there is no orderly separation of pairs or sets of chromosomes. You can’t evenly divide an odd number. Most reproductive cells will have an odd assortment of chromosomes and the cells (eggs and sperm) will not function normally. We expect the plant to be sterile.
The cross between P. vitifolium (2n = 88) and P. capitatum (2n = 66) has been made. Offspring are produced but are sterile as one would expect of a heptaploid (seven sets of chromosomes, n = 44 plus n = 33 yields a plant with 2n = 77). A similar origin has been proposed for ‘Rosé’ a rose-scented cultivar commercially important in the production of geranium oil. This cultivar is a hybrid between P. capitatum (2n = 66) and P. radens (2n = 88) as parents. One would expect this plant to be sterile too (and they normally are). The cultivar ‘Kelker’ (important in India for geranium oil production) has been studied and its origins are similar. In the case of the clone ‘KF-1’, a fertile plant arose from the vegetatively propagated cultivar ‘Kelker’. There was a doubling of the heptaploid chromosome number (from 2n = 77 to 4n = 154) so now at meiosis each set of chromosomes has an identical partner with which to pair and meiosis proceeds normally.
There are many other aspects of chromosomes to be considered in the evolution of the Geraniaceae. These will be investigated in a future article. [If you are interesting in reading the original references on which this article is based please e-mail the author