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Animation: Polyploid Plants

by Pearson
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It is possible for new species to arise quickly and spontaneously, through genetic isolation, without geographic isolation. This process occurs in a variety of ways, mostly in plants. Imagine a plant - let's call it species A - with a diploid chromosome number of 6. Normal meiosis in this species produces haploid gametes gametes with one set of chromosomes apiece. Each gamete contains 3 chromosomes. Note that for clarity we are greatly simplifying meiosis. Self-fertilization results in a diploid zygote with 6 chromosomes that grows into a normal diploid adult. Virtually overnight, a chromosomal mutation can create a genetic barrier between mutants and a parent population, leading to the appearance of new species. This happens because an error in meiosis creates gametes with two sets of chromosomes - 6 chromosomes instead of the normal 3. This zygote would have a total of 12 chromosomes. Extra sets of chromosomes are termed polyploidy. This particular zygote is tetraploid, because it has 4 sets of chromosomes, instead of the usual 2 sets. The zygote could develop into a tetraploid plant. In plants, such polyploids are often viable and even vigorous. In this individual, all 4 sets of chromosomes were derived from a single parent species. This triploid plant would produce gametes containing 6 chromosomes. Offspring would have 12 chromosomes, like their parents. So, the tetraploid plant could produce offspring but what makes these plants a new species? If pollen from the tetraploid (12 chromosomes)plant fertilized eggs from the original diploid (6 chromosomes) plants, triploid zygotes with a total of 9 chromosomes would result. Such a zygote could undergo mitosis, growing into a triploid individual. But, the triploid hybrid plant would be sterile. Homologous chromosomes are supposed to pair up in meiosis. In the triploid, some of the chromosomes would lack partners, meiosis would go awry, and no gametes would be produced. Thus, the mutant tetraploid plants are self-fertile, but unable to interbreed with the original diploid plants. In one generation, a reproductive barrier has isolated the new tetraploid plants, a new species, from the original diploid species. Polyploids derived from a single parent species are relatively rare. Most polyploids arise from the hybridization of two different parent species. Imagine pollen from a species with a diploid number of 4 fertilizing the eggs of a species with a diploid number of 6. We will call them species W and X. The cells of the resulting hybrid plant would have 5 chromosomes -- 2 plus 3. And the hybrid would be sterile. The mismatched chromosomes would be unable to pair in meiosis, and no gametes would be produced. Occasionally, a hybrid produces gametes with the same number of chromosomes as the hybrid itself. If self-fertilization occurs in the example shown here, the plant that develops from this zygote would be fertile and would able to carry out meiosis. The chromosomes could pair, and meiosis would occur normally, producing gametes containing 5 chromosomes apiece. Self-fertilization would produce offspring whose cells have a diploid number of 10. Thus, polyploidy can produce a new plant species species Z which is reproductively isolated from the parent species because of their differing numbers of chromosomes.
It is possible for new species to arise quickly and spontaneously, through genetic isolation, without geographic isolation. This process occurs in a variety of ways, mostly in plants. Imagine a plant - let's call it species A - with a diploid chromosome number of 6. Normal meiosis in this species produces haploid gametes gametes with one set of chromosomes apiece. Each gamete contains 3 chromosomes. Note that for clarity we are greatly simplifying meiosis. Self-fertilization results in a diploid zygote with 6 chromosomes that grows into a normal diploid adult. Virtually overnight, a chromosomal mutation can create a genetic barrier between mutants and a parent population, leading to the appearance of new species. This happens because an error in meiosis creates gametes with two sets of chromosomes - 6 chromosomes instead of the normal 3. This zygote would have a total of 12 chromosomes. Extra sets of chromosomes are termed polyploidy. This particular zygote is tetraploid, because it has 4 sets of chromosomes, instead of the usual 2 sets. The zygote could develop into a tetraploid plant. In plants, such polyploids are often viable and even vigorous. In this individual, all 4 sets of chromosomes were derived from a single parent species. This triploid plant would produce gametes containing 6 chromosomes. Offspring would have 12 chromosomes, like their parents. So, the tetraploid plant could produce offspring but what makes these plants a new species? If pollen from the tetraploid (12 chromosomes)plant fertilized eggs from the original diploid (6 chromosomes) plants, triploid zygotes with a total of 9 chromosomes would result. Such a zygote could undergo mitosis, growing into a triploid individual. But, the triploid hybrid plant would be sterile. Homologous chromosomes are supposed to pair up in meiosis. In the triploid, some of the chromosomes would lack partners, meiosis would go awry, and no gametes would be produced. Thus, the mutant tetraploid plants are self-fertile, but unable to interbreed with the original diploid plants. In one generation, a reproductive barrier has isolated the new tetraploid plants, a new species, from the original diploid species. Polyploids derived from a single parent species are relatively rare. Most polyploids arise from the hybridization of two different parent species. Imagine pollen from a species with a diploid number of 4 fertilizing the eggs of a species with a diploid number of 6. We will call them species W and X. The cells of the resulting hybrid plant would have 5 chromosomes -- 2 plus 3. And the hybrid would be sterile. The mismatched chromosomes would be unable to pair in meiosis, and no gametes would be produced. Occasionally, a hybrid produces gametes with the same number of chromosomes as the hybrid itself. If self-fertilization occurs in the example shown here, the plant that develops from this zygote would be fertile and would able to carry out meiosis. The chromosomes could pair, and meiosis would occur normally, producing gametes containing 5 chromosomes apiece. Self-fertilization would produce offspring whose cells have a diploid number of 10. Thus, polyploidy can produce a new plant species species Z which is reproductively isolated from the parent species because of their differing numbers of chromosomes.