RE: Re; Hormesis or not ( number of chromosomes )

["Bjorn Cedervall" <bcradsafers@HOTMAIL.COM>]

>Sorry to hark back to an old thread, but the question of the reaction of 

>plants to radiation reminded me that a plant geneticist once told me that 

>the irradiation of plant seeds can effect a doubling of the chromosomes 

>(visible under a microscope), and that such plants with a double set 

>usually have superior properties. Could that be termed a hormetic effect?

----



This seems to be about mechanisms in evolutionary biology regardless of the 

cause.



A doubling of the number of chromosomes (from the diploid state with two 

sets, 2n) can probably occur in a few different ways:

After a normal doubling of the number of chromosomes there is a "cell cycle 

checkpoint" to make sure that there normally is no second doubling after the 

first one without first going through cell division. If such an error occurs 

and is transmitted to the next generation it must be related to the meiotic 

events forming the germ line cells.

More common is certainly hybridization where two closely related species 

fuse to form a tetraploid (4n) or a tetraploid and a diploid fusing to form 

a hexaploid (6n) and so on. G L Stebbins had some publications in the late 

1940s relating to the complex ploidy patterns in the grass genus Bromus (for 

a summary, See CP Swanson, Cytology and Cytogenetics, 1968, pp.511-512)



A higher ploidy level allows for a higher degree of heterozygosity which 

means that one individual can carry more of the genetic variation. This in 

turn may be an advantage. Changes in karyotype (number of chromosomes) is 

part of common evolutionary mechanisms - in particular among plants. Among 

flowering plants about 40 % or more are polyploid (4n, 6n, 8n, 10n etc). 

Some extreme examples are found among "ancient taxa" like ferns 

(Ophioglossum reticulatum: 1260 chromosomes) and horse tails (always 216 

chromosomes in the zygote - the old diploid ancesters became extinct perhaps 

more than hundred million years ago ). In contrast, Ginkgo biloba (a living 

dinosaur among plants) and cycads do probably not have any polyploid forms. 

The number of chromosomes is a complex topic and sometimes numbers that may 

seem unexpected are found: Rosa canina (rose complex in northern Europe) as 

a pentaploid was observed already about 80 years ago. How did we get 46 

chromosomes? (I should know but haven't checked the current opinion - a 

guess is that it comes from 48 chromosomes in some ancestor which can be 

broken down backwards in time by numerical splittings but a number like 46 

could also just be where we are after successive splitting of chromosomes 

one by one. The evolutionary significance is about keeping genes together 

(linkage groups) during chromosomal reassortment in meiosis).



An increase in the ploidy level immediately creates a hydridization barrier 

(genetic isolation). Therefore, if such offspring survives it represents 

instantaneous speciation. Polyploids are rare among animals.



The effects on irradiated seeds can be difficult to interpret unless a 

detailed analysis is performed on a DNA level. Åke Gustafsson published 

several papers around 1946-1955 (approx) about irradiated barley seeds and 

found some improvements in the later generations. This was interpreted in 

terms of cooperation of several genes (it was not about changes in 

chromosomal number) - an idea that T. Dobhansky put on the scene in 1937. 

The beneficial mutations that Gustafsson obtained may have been in the order 

of more than 5 % of all mutations - an interpretation I made while reading 

some related text (Stebbins 1950??).



Any experimental work must be recognized as such. The real situation with a 

free flow of alleles from the rest of the gene pool plus environmental 

variations may result in very different results since these latter factors 

in the short term usually are much more important than new mutations or 

chromosomal rearrangement.



Evolutionary biology is a good reference frame to understand and discuss 

phenomena relating to changes of DNA and chromosomes. Evolution is only 

fought by some extremists of various kinds.



My personal ideas only - please add or subtract whatever may be necessary,



Bjorn Cedervall   bcradsafers@hotmail.com

http://www.geocities.com/bjorn_cedervall/



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