Polyploidy Modified from PP found at /evollect04-7.ppt

This is the chromosome complement of the fern Ophioglossum, the organism with the highest known chromosome number (N = 630, 2N = 1260). Note: This drawing was published in It was made with a camera lucida, an apparatus that is no longer common.

“Polyploidy:” Changes in the number of whole genomes or chromo- some sets. Probably results from failures of cytokinesis after chromosome replication during cell division. Sometimes called “euploidy.” Why is polyploidy important? 1.Very common in some groups. Upwards of 70% of angiosperms (flowering plants) are polyploids--some botanists estimate 90%. 2. Sometimes can be a form of “instant speciation.” 3. Some polyploids are widely used in agriculture or industry.

How can polyploidy sometimes result in “instant speciation?” AA (ancestral diploid) AAAA (autotetraploid) AA (ancestral diploid) gametes: AA A AAA triploid hybrid (usually sterile) Because the hybrid is sterile, the tetraploid is, in effect, a new species, for there will be little gene flow between it and the ancestral diploid. That is, it is genetically isolated from its diploid ancestor. genome doubling

Some properties of polyploids: 1.Odd-ploids (e.g., triploids) are almost always sterile. See if you can figure out why this might be so… 2.Even-ploids (e.g., tetraploids) are frequently fertile, but not always… 4.Because they have larger nuclei, poyploid cells tend to be larger than haploid or diploid cells, and cell size (or volume) increases with ploidy level. The last property is important in two agricultural/industrial applications

Some properties of polyploids, contd: 5. Flowers and fruits of polyploid plants are usually relatively larger than those of their diploid ancestors. This has obvious agricultural implications, but also may be important in natural populations (e.g., a polyploid may produce more pollen or seeds). 6.Polyploids are more common in plants than in animals. 7.Among plants, polyploids are most common in wind-pollinated species.

Some properties of polyploids, contd: 5. Flowers and fruits of polyploid plants are usually relatively larger than those of their diploid ancestors. This has obvious agricultural implications, but also may be important in natural populations (e.g., a polyploid may produce more pollen or seeds). 6.Polyploids are more common in plants than in animals. Why might this be so? Ans: Plants can reproduce asexually, so, for example, a sterile hybrid (e.g. an AB diploid) can persist until fertility is restored by genome doubling. 7.Among plants, polyploids are most common in wind-pollinated species. Why might this be so? Ans: Wind-pollination can more easily lead to hybridization, a requirement for allopolyploidy.

Polyploids are often of commercial importance, especially in plants. Polyploid plants are often larger (taller) than their diploid ancestors. In addition, reproductive structures (flowers, fruits, etc.) tend to be proportionally larger than stem tissues.

Bread Wheat, Triticum aestivum, is an allohexaploid species, some of its diploid and tetraploid ancestors were cultivated (“c”). It took many years to identify the “D” genome and its precise identity is still controversial. C? c c This is our modern bread wheat. A hexaploid, its seed heads (our grain) are much larger than the other forms. Something to think about: How much of the evolution of wheats happened naturally, and how much was guided by us? 2N = 42 = 6X or 2N = 42, X = 7 X = haploid no. of the ancestral species.