Diploidy or not diploidy. That is the question...

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Re: Diploidy or not diploidy. That is the question...

Post: # 58815Post Warren
Sat Dec 20, 2014 4:12 pm

one thing you must look at is, diploids only have half the chromosomes therefore less of the bad traits to be represented.

Karl K
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Re: Diploidy or not diploidy. That is the question...

Post: # 58893Post Karl K
Tue Dec 30, 2014 9:26 am

Don wrote:>> The polyploid species are allopolyploids, combining chromosomes of two or more distinct diploid species.

You stopped me cold here, Karl. Can you tell me why you say this?

I have learned that I was premature in asserting that autopolyploids have no selective advantages in the wild. Sometimes they do ... though usually not in areas where the species is comfortable. The advantages will more often be found at the limits of a species' range, particularly where pushing northwards or upwards (in mountains). Tetraploid races of diploid species are often slower growing (one way to avoid late frosts); and sometimes (e.g. tomatoes) higher in vitamin C. Ascotbic acid and ascorbates are antioxidants that protect plants against frost damage.

The Evolutionary Significance of Autopolyploidy
Arne Müntzing

So, where a diploid species is comfortable, tetraploid forms gain no great advantage and are soon lost. But where two diploid species are present and both are fairly comfortable, hybrids between the two may give rise to happy tetraploids -- if the chromosomes of the two species are sufficiently differentiated.

Then, if a species is migrating upwards or northwards, autotetraploids can gain physiological advantages over the diploid parent species.

And then, if two diploid species are migrating in the same direction, and each pushes forward by doubling their chromosomes, the autotetraploids can then cross to form new allotetraploid species.

Or, an autotetraploid race of one species entering a new (to the species) habitat may encounter a different diploid species that is already native to the region. Triploid hybrids result, which may then produce allotetraploid progeny. Or the diploid may generously provide an unreduced gamete, giving an allotetraploid hybrid immediately.

Allotetraploids (and higher polyploids) gain a greater advantage over autotetraploids through increased phenotypic flexibility by modulation of multiple gene-sets.

Now, how does a higher polyploid break down and produce lower levels of ploidy? They must, or they will be trapped in the far north or at high altitudes.
New Phytol. 37: 72-81. 1938
A plant of octoploid R. acicularis from Alaska was transferred to Ann Arbor, Michigan, where it leafed out soon after the first April thaw and was frequently badly damaged by frost in May. In some seasons all the floral primordial tissue was destroyed and no flowers were produced. The alternating thaws and frosts which are a feature of continental climates between 40 and 50° of latitude North, may militate against the spread southwards of octoploid races in Rosa. The Alaskan octoploid thrived at Pasadena (Erlanson, 1934) in southern California, and came into flower early in February, 3 1/2 weeks before the hexaploid type.
http://bulbnrose.x10.mx/Roses/breeding/ ... y1938.html

Obviously, an Alaskan octoploid will not migrate to Pasadena without passing through areas where it is not likely to survive long enough to reproduce. To gain the ability to survive at lower latitudes (or lower altitudes) it must lose some of its chromosomes. This might be achieved by crossing with different species with lower chromosome numbers (if the species are available) or by being pollinated by plants of other genera.

Or, the higher polyploids may become mitotically unstable.
Journal of Heredity 48: 11-20 (1957)
In one population of 66 seedlings of Eldorado (4x), (2n=28) selfed, five of the young plants were determined to be 2n=42 by means of axillary leaf-bud squashes. These 6x plants presumably resulted from fertilization of unreduced eggs by reduced pollen grains—an event not uncommon in Rubus. In contrast to the regular-appearing leaves of 4x Eldorado selfed seedlings (Figure 5), the 6x seedlings exhibited, in addition to their symptoms of higher ploidy a noticeable degree of "checking" or mosaicism in their leaves (Figure 5B) Cytological examination of their root-tips indicated that many cells had 42 chromosomes and others had fewer—some markedly fewer (Table II). No evidence of mitotic instability has been found in 4x Eldorado plants or 4x selfed seedlings of Eldorado.
http://bulbnrose.x10.mx/Heredity/Britto ... ility.html

The point, here, is that higher polyploids are not evolutionary dead-ends forever trapped in the far north or at high altitudes. They may give rise to lower polyploids and even diploids. These derived diploids will not be identical to the original diploid parents, but will often possess a "family resemblance". Likewise for tetraploids derived from hexaploids or octoploids.

Karl K
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Joined: Sat Jun 02, 2012 4:49 pm

Re: Diploidy or not diploidy. That is the question...

Post: # 61356Post Karl K
Thu Jul 16, 2015 6:54 am

I gave the wrong link above:

The Evolutionary Significance of Autopolyploidy
Arne Müntzing

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Joined: Thu Dec 15, 2011 10:28 pm

Re: Diploidy or not diploidy. That is the question...

Post: # 67962Post philip_la
Sun Jun 10, 2018 11:31 am

In view of a recent post, I thought I might just bring this old thread back to the surface.
Philip F.
[size=small][color=#669966]Zone 8 / Sunset Zn 30 (Austin, TX -- formerly New Orleans, LA)[/color][/size]

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