Hi, 

Pl go through the following chapter: 

Morishima H (1984) Wild plants and domestication. Chapter 1. In: S Tsunoda, N Takahashi, eds. Biol Rice. 7: 3-30.

In cultivated plants there are chances of ploidy levels. Ploidy differences sometimes results in expression of desired characterrs and through selection they become more accepted.  If it is Euploidy, normal sexual propagation  will be possible, but in aneuploidy  normal sexual reproduction become difficult. Many Solanaceae plants such as Capsicum shows ploidy differences.

For speciation there are many reasons and ploidy difference  may also contribute,  but where expression of characters are amplified one can  also suspect  false speciation. Here a thorough knowledge and true assessment is needed. For example the diverse characters expressed in banana  (Musa) due to ploidy differences are never considered as speciation,

Hi Binoy

Just take the example of wheat. The cultivated wheat T. aestivum, is an allohexaploid (allopolyploid) with 6 sets of chromosomes (14+14+14). Two sets come from T. urartu and 2 sets from Aegilops speltoides (= A. squarrosa). This formed T. turgidum (durum wheat); i.e., T, turgidum= (T. urartu x A. speltoides) + doubling of chromosomes. The last two sets came for A. tauschii; i.e., T. aestivum = (T. turgidum x A. tauschii) + doubling of chromosomes. This chromosomes pair at meiosis in 21 pairs (7 + 7 + 7) and then each gamete gets a full set of 21 chromosomes.

In this case, as in many other allopolyploids, ploidy levels are associated with speciation because the hybrids are sterile: if you cross T. aestivum x T. urartu, you get at meiosis 7 pairs + 14 non-paired chromosomes, and these late ones will form aneuploid gametes. On the other hand, if you cross T. aestivum x S. cereale = 21 +7 chromosomes, you get a sterile plant. But if you then double their chromosomes, you get Triticale, which is a new fertile species.

Speciation is not that easy to explain for autopolyploids (i.e., Secale montanum, Zea perennis, etc.). But just imagine an autotetraploid formed by doubling of chromosomes of some species. If you cross the autotetraploid and the diploid you get triploids, which will be sterile. According to the biological species definition, two organisms are the same species if they form fertile hybrids. Therefore, your diploid and tetraploid organisms are different species.

Hope this helps

Pablo

This is a very broad question and the answer depends on the plant species. Wild relatives may have different levels of ploidy and cultivated forms can either have higher or lower levels of ploidy, again depending on the species. For instance, cultivated potatoes are tetraploid (mostly) but  you have wild relatives that go from diplods to hexaploids. Generation of poly- or disomic polyploids respond to diverse hypothesis that go from genome duplication to genome capture, there are several books/papers presenting this and again every plant species has its own history on these processes. 

Very simply, yes.  There are quite number of examples but, as Alfonso, suggests this is quite a broad question and you may wish to narrow to a specific genus or family.

wheat, peanuts, some brassicas are the classic examples where the wild parents are of different ploidy to the cultivated ones

Dear Binoy

Cultivars emerged as a result of careful selection and breeding practices of farmers and unknowingly polyploids have often been favoured for their beneficial characters. Compared to their wild relatives polyploids are often larger in size and produce bigger fruits sometimes without seeds (banana), more grains or larger flowers (horticulture) which is commercially beneficial.

Formation of polyploids may be triggered due to normal meiotic failure resulting in diploid gamete which then fuse with normal haploid ones producing triploid zygotes, which may become sterile or contribute to further anomalies in gamete formation promoting polyploid. Alternately fusion of diploid gametes leads to tetraploid plants which are more stable than the triploids.

 Most common polyploids that we encounter are bananas (mostly triploid), wheat (hexaploid) and potato (tetraploid).

A common misconception is that most, or significantly more than expected, domesticated plants are polyploid, however this isn't the case:

http://www.jstor.org/stable/2445679?seq=1#page_scan_tab_contents

Polyploidy is however a major force in generating new species:

http://www.pnas.org/content/106/33/13875.full

In my plant breeding activities in the past, in ornamental plants, I hybridised diploid species of Anigozanthos (kangaroo paws), crossing the vigorous disease resistant 'indestructible'   A. flavidus with several other disease prone or short lived diploid species e.g.  A. manglesii, A. humilis, A. viridis, A. pulcherimus,  and so on. The diploid hybrids were all strerile, but I restored fertility by converting the diploid hybrids to tetraploids by adding colchicine to the medium in tissue culture. With this restored fertility I was able to cross the various allotetraploid hybrids among themselves to yeild allotetraploid hybrids, derived from three species, e.g. 1/2 A.flavidus, 1/4 A. humilis, 1/4 A. viridis,  or, 1/2 A. flavidus, 1/4 A. humilis, 1/4 A. pulcherimus, and so on.  Many plant breeders do very similar things to this, so often cultivars are hybrid tetraploids  (or higher polyploids) which have have been derived from diploid 'wild type' parental species.

Other high ploidy levels in cultivars could be derived  from low ploidy wild type species or hybrids by plant breeders, so cultivars often have a higher ploidy level, than their wild type progenitors, and indeed wheat is hexaploid, for example.

In the wild, natural polyploidy can facilitate speciation, or reproductive isolation, within a genus, making polyploidy a significant driver of evolution in angiosperms, but gymnosperms tend not to produce polyploids, and tend not to evolve much. There are, of course, other facilitators of evolution, such as transposable elements (TEs) interacting with polyploidy and other factors, driving evolulution in angiosperms.

In natural contact zones of closely related diploid and tetraploid species of North America,  triploids are often formed. Moreover there is strong evidence indicating that such populations contain tetraploids that have characteristics more commonly associated with the diploid species.  Moreover some of the triploids in these contact zones are semifertile.  Although the evidence is not conclusive there are diploids in these populations than show signs of introgression from the tetraploid species. 

In other words there seems to be evidence indicating bidirectional introgression between the diploid and tetraploid species.  The triploids may acting as a two way bridge.