Ploidy Varation in Plants.pdf

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Ploidy Variation in PlantsJames A Birchler, University of Missouri, Columbia, Missouri, USA

Ploidy variation involves changes in the number of whole sets of chromosomes. Aneuploid

variation involves changes in the number of individual chromosomes. The two types of

chromosomal changes have different consequences for gene expression patterns.

Polyploidy

Polyploidy is the situation in which there are multiplecopies of the genome present in a cell, an individual or aspecies. The term basic chromosome number (x) refers tothe number of chromosomes in a complete set, also knownas a genome. The gametic chromosome number (n) is thenumber of chromosomes in the gametophyte and thegametes. The term haploid refers to the gametophyte andcases in which the gametic numberis present in sporophytic

tissue. The term monoploid refers to individuals with asingle genome in the sporophyte. The most commonploidies are monoploid (1x), diploid (2x), triploid (3x),tetraploid (4x), pentaploid (5x), hexaploid(6x), heptaploid(7x) and octoploid (8x).

In diploid species, the haploid has a single genomepresent and is referred to as a monoploid. Haploids havebeen found in most taxonomic groups. They are smallerand have reduced floral organs compared to diploids.Haploids can arise via the parthenogenic development of acell in the megagametophyte, usually thought to be the eggor a synergid (Kimber and Riley, 1963). X-irradiation,delayed pollination and interspecific crosses can also

stimulate the production of haploids. In maize, haploidscan be generated by a certain inbred line (stock 6) thatproduces gynogenetic or maternal haploids in several percent of the progeny (Coe, 1959). The indeterminategametophyte (ig) mutation conditions paternal or andro-genetic haploids (Kermicle, 1969).

Monoploids are predicted to give complete genomecomplements in the gametes at a frequency of (1

2)n, where n

is the gametic number of chromosomes. Consequently,most spores (the endproductof meiosis) are missing one ormore chromosomes and abort. However, normal gameto-phytes occur at greater frequencies due to spontaneouschromosome doubling in premeiotic tissue, followed by

normal meiosis, which produces functional spores.One-third to one-half of all plant species are polyploids

of some type; however, in some cases, ancient polyploidshave evolved to the point that they behave as a diploid, as isthe case with maize. There are two types of polyploids.Autopolyploids contain multiples of the same genome,whereas allopolyploids (also called amphiploids or am-phidiploids) are derived from two or more differentgenomes. Allopolyploidy is more common than autopoly-ploidy.

Triploids can be produced from crosses betweetetraploid and diploid plants or by fertilization of diploifemale gametes, which result from failed meiosis. Thmeiotic behaviour of triploids has been studied extensivelin Jimson weed (Datura) and in maize (McClintock, 1929Satina et al., 1938; Punyasingh, 1947). The distribution ochromosomes in meiosis is nearly random. The resultinchromosome numbers range from the 1x to the 2x leveThe resulting aneuploidy causes seed abortion, whicexplains the nearly complete sterility of triploids. Banana

and seedless watermelon derive their sterility from the facthat they are triploids.Most polyploid plants are tetraploids. In allotetra

ploids, two different genomes are present. They arise fromcrosses between diverged species followed by chromosome doubling. Because the two sets of chromosomehave diverged, the pairing in meiosis occurs betweechromosomes that are most similar. This situation resultin bivalent formation in prophase of meiosis anconsequently in so-called disomic inheritance. In othewords, segregation in allotetraploids is the same as idiploid species, although most characteristics are controlled by duplicate genes, because two genomes ar

present. A recessive allele at both loci must be present fothe corresponding phenotype to result. The identical pairof homologues will preferentially pair with each other imeiosis rather than with the homologous chromosomesi.e. the partially homologous chromosomes derived fromthe two progenitor species.

In cases in which the chromosome number of a diploihas been doubled, an autotetraploid is formed. When thfour chromosomes synapse in meiosis they form quadrivalent and exhibit tetrasomic segregation. Ireferring to tetraploid segregation, the homozygoudominant, AAAA, is a quadriplex and the homozygourecessive, aaaa, is a nulliplex. The three types of hetero

zygotes are: triplex, AAAa; duplex, AAaa; and simplexAaaa.Autotetraploid segregation is affected by recombinatio

between the locus being followed and the centromerWhen exchange occurs between chromosomes carryindifferent alleles, the resulting products will be twheteroallelic chromosomes and two homoallelic chromosomes. If the heteroallelic chromosomes proceed to thsame pole in meiosis I, then in meiosis II both recessivmarkers could come together in the same meiotic produc

Article Contents

Secondary article

. Polyploidy

. Aneuploidy

. Polyteny and Endoreduplication

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is a direct effect in which there is a direct correlation ofgene expression with the dosage of the varied segment, thatis, a reduction to 50% of normal in monosomics and anincrease to 150% in trisomics compared with the normaldiploid. Any one structural gene is typically affected byseveral aneuploid series (Birchler and Newton, 1981; Guoand Birchler, 1994).

The inverse effect appears to be the underlying basis ofthe dosage compensation found in larger aneuploids(Birchler, 1981). The inverse effect is of the propermagnitude to cancel a gene dosage effect when such atrans-acting regulator and its target structural gene arevaried together. The combination of a reduction to 50%resulting from a structural gene dosage effect and a 200%increase of a trans-acting inverse effect in a monosomicresults in an expression similar to the diploid. Likewise anincrease to 150% in a trisomic is cancelled by a reductionof each of the three copies to 67% of the normal level ofexpression. Dissection of large aneuploids that exhibitdosage compensation of alcohol dehydrogenase in maize

shows that smaller aneuploids surrounding the structuralgene exhibit a dosage effect, and another segment of thelarger aneuploid produces an inverse dosage effect, on Adh(Birchler, 1981). Thus, the basis of the compensationfound in larger aneuploids is the result of a combination ofa dosage change of the structural gene and a trans-actingregulatory gene, which inversely affects the structuralgene.

The changes (or lack thereof) in gene expression inaneuploids described above illustrate a much morecomplex pattern than mere dosage effects of the variedgenes. Indeed the majority of the varied genes are dosagecompensated, although a substantial fraction escape this

process to produce a dosage effect (Guo and Birchler,1994). Much more dramatic effects occur in the unvariedportion of the genome. The direct and inverse effectsproduce decreases and increases in both monosomics andtrisomics.

In contrast to the extensive changes in gene expressionthat occur in aneuploids, variation of the whole genomeproduces less extreme effects (Guo et al., 1996). A study ofmessenger ribonucleic acid (mRNA)levels from 18 genes ina monoploid, diploid, triploid and tetraploid series inmaize showed that the predominant response in a ploidyseries is an expression per cell that is directly proportionalto the ploidy level (Guo et al., 1996). Indeed, cell size

correlates with ploidy, suggesting that the increase incellular components will produce a larger cell; however,other patterns occur. The major exceptional effect is one inwhich there is a positive correlation with ploidy that isgreater than predicted fromthe change in ploidyalone.Theminor exceptional effect is a negative correlation withploidy.

Polyteny and Endoreduplication

In specific tissues in some species, deoxyribonucleic aci(DNA) replication has continued without cell division tcreate nuclei with many copies of the chromosome setWhen the entire genome is involved and the replicatechromosomes remain aligned, this situation is referred t

as polyteny. Several instances have been documented iplants, most notably in suspensor cells of several species obeans (e.g. Nagl, 1969). In the endosperm of maizeportions of the genomeare overreplicatedto produce mancopies in the cell, a situation referred to as endoreduplication (Grafi and Larkins, 1995). In several species thapossess small genome sizes, all portions of the chromosomes are endoreduplicated in a developmentally controlled manner (DeRocher et al., 1990).

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polyploidy. Critical Reviews in Plant Sciences 12: 243273.

Stebbins GL Jr (1947) Types of polyploids: their classification an

significance. Advances in Genetics 1: 403429.

Ploidy Variation in Plants

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