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The Advantages and Disadvantages of Being Polyploid

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The Advantages and Disadvantages of Being Polyploid REVIEWS THE ADVANTAGES AND DISADVANTAGES OF BEING POLYPLOID Luca Comai Abstract | Polyploids — organisms that have multiple sets of chromosomes — are common in certain plant and animal taxa, and can be surprisingly stable. The evidence that has emerged from genome analyses also indicates that many other eukaryotic genomes have a polyploid ancestry, suggesting that both humans and most other eukaryotes have either benefited from or endured polyploidy. Studies of polyploids soon after their formation have revealed genetic and epigenetic interactions between redundant genes. These interactions can be related to the phenotypes and evolutionary fates of polyploids. Here, I consider the advantages and challenges of polyploidy, and its evolutionary potential. NEOPOLYPLOID Polyploidy is the heritable condition of possessing that these processes are often not random and that the A polyploid that has been more than two complete sets of chromosomes. Most function and properties of the encoded protein affect produced by artificially polyploids have an even number of sets of chromo- the outcome18–25. By providing duplicated genes, poly- inducing chromosome somes, with four being the most common (tetraploidy). ploidization might fuel long-term diversification and doubling. Polyploids are very common among plants and com- evolutionary success. DIPLOIDIZATION mon among fish and amphibians, and are usually fit and In this review I discuss possible advantages of Gradual conversion from well adapted. Indeed, the study of eukaryotic genomes polyploidy and constraints on polyploid formation polyploidy to diploidy through is providing surprising proof of the evolutionary poten- that are indicated by either experimental evidence or genetic changes that tial of polyploids: many sequenced genomes display theoretical considerations. Becoming and remaining differentiate duplicated loci. the signature of polyploidy ancestry1–8. This indicates polyploid changes the organization and function of SUBFUNCTIONALIZATION that polyploidy can bestow long-term evolutionary the genome at both genetic and EPIGENETIC levels. For Retention by duplicated genes flexibility, instead of freezing species in a static state example, in addition to the creation of gene redun- of different components of the that is enforced by gene redundancy, as was originally dancy, polyploidy causes nuclear enlargement and original common function. proposed9. increases the complexity of the processes that are NEOFUNCTIONALIZATION In fact, polyploidy can be advantageous. On the basis involved in managing and partitioning chromosomes Acquisition of novel function by of the phenotypic and molecular characterization of during cell division. Perhaps the most striking evi- a duplicated gene. NEOPOLYPLOIDS, it has been inferred that after polyploids dence of change comes from the discovery of epige- form they pass through a bottleneck of instability10–13, netic remodelling, which leads to both the activation before becoming adapted and joining the evolutionary and suppression of gene expression. Although some Department of Biology, Box 355325, University of fray as efficient competitors of their diploid relatives. of these changes are potentially advantageous, many Washington, Seattle, Adapted polyploids that avoid extinction enter an cause instability of the neopolyploids and might be Washington 98195, USA. evolutionary trajectory of DIPLOIDIZATION, during which disruptive. This review of how polyploids form and e-mail: comai@u. genomic redundancy is reduced14,15. Duplicated genes adapt will not only be useful to readers who are inter- washington.edu doi:10.1038/nrg1711 can be lost, retained or maintained as duplicates, often ested in the evolutionary role of polyploids. It will also Published online undergoing SUBFUNCTIONALIZATION and NEOFUNCTIONALIZA be useful to those who are interested in how parental 11 October 2005 TION16,17. Bioinformatic and theoretical analyses indicate gene interactions can lead to HETEROSIS or DYSGENESIS, 836 | NOVEMBER 2005 | VOLUME 6 www.nature.com/reviews/genetics © 2005 Nature Publishing Group REVIEWS the type and the origin are different. This is because Diploid autopolyploidy results from a mutation in chromo- some number whereas allopolyploidy results from Diploidy concurrent hybridization and mutations in chromo- Speciation some number. The total number of chromosome sets is Diploid species AA Diploid species BB indicated by the prefix; for example, tri- (3), tetra- (4), penta- (5), hexa- (6) and octa- (8). Meiotic pairing arrangements vary between ploidy 2N gamete types (FIG. 2). In particular, the increased complexity of 2N + 1N gametes 2N gamete pairing can cause the deletion or addition of chromo- somes from the balanced complement that is expected F1 (AB) in the gametes (FIG. 2b). The division between autopoly- 2N gamete ploids and allopolyploids is not absolute. The chromo- Duplication some sets of allopolyploids differ proportionally to the divergence of the parental genomes: the closer the Triploid (AAA) 2N + 2N parents, the more similar the resulting allopolyploid is gametes to an autopolyploid. This potential ambiguity between classes of polyploidy has been addressed by proposing 27 Polyploidy a third class, segmental allopolyploidy , but the utility Autotetrapoloid Allotetrapoloid of this category is questionable. (AAAA) (AABB) An immediate consequence of polyploidy is the Partially diploidized tetraploids change in gametic and filial frequencies. Mendel’s rules of segregation and assortment still apply but there is an increase in the number of ‘‘factors’’ to consider. This Diploidization Diploid produces ratios that are different from those that were observed by Mendel in diploid peas12 (FIG. 3). Polyploids form at relatively high frequency (1 per 100,000) in flowering plants26. Because of this high Diploidy formation rate and the polyploidy tolerance of plants, stable polyploidy is common in plants. The frequent Figure 1 | Evolutionary alternation of diploidy and polyploidy. The figure shows the 28 possible paths that result in the sudden transition from diploidy to polyploidy and the gradual occurrence of stable polyploidy in fish and frogs indi- transition from polyploidy to diploidy. The hybridization events that result in ALLOPOLYPLOIDY are cates that the formation of polyploids is also possible in also illustrated. For simplicity, not all possible paths are drawn. Triploids, for example, are animals, even if it is only stable in certain animal taxa. shown contributing to autotetraploids but they can also contribute to allopolyploids. For each Indeed, higher vertebrates do not tolerate polyploidy, ploidy form, the haploid genome is represented by a coloured circle or oval inside the beige- but 10% of spontaneous abortions in humans are due filled nuclear shape. Genomes that are illustrated by ovals reflect the increased gene number to polyploidy29. that results from the retention and subfunctionalization of duplicates during diploidization. Circles or ovals of different colours represent diverged genomes. Highly unstable ploidy forms have dashed nuclear contours. A and B represent genome types and N is the gametic Advantages of polyploidy chromosome number. There are three documented or obvious advantages of becoming polyploid. The first two, heterosis and gene redundancy, are the result of gene duplication, whereas in how epigenetic patterns are maintained or altered, the mechanistic connection to polyploidy of the third, and in how structural features of genomic organiza- asexual reproduction, is unclear. Heterosis causes poly- tion affect cellular functions. ploids to be more vigorous than their diploid progeni- EPIGENETIC tors, whereas gene redundancy shields polyploids from A mitotically stable change in gene expression that depends The mechanics of polyploidy the deleterious effect of mutations. Asexual reproduc- not on a change in DNA Formation and incidence of polyploidy. Polyploids are tion enables polyploids to reproduce in the absence of sequence, but on covalent divided into categories depending on their chromosomal sexual mates. modifications of DNA or composition and their manner of formation. Polyploids chromatin proteins such as histones. arise when a rare mitotic or meiotic catastrophe causes Heterosis. Polyploids take advantage of heterosis in at the formation of gametes that have more than one set of least three ways. One involves the fixing of divergent HETEROSIS chromosomes (FIG. 1). Diploid gametes, which arise infre- parental genomes in allopolyploids. Whereas heterozy- The increase in performance quently, typically fuse with haploid ones and produce tri- gosity and heterosis decay in the progeny of a diploid displayed by hybrids ploid zygotes, which are unstable and can either be sterile F1 hybrid (at each generation half the heterozygous compared with their inbred parents. Because performance or contribute to further polyploid gametes, depending loci become homozygous), the enforced pairing of 26 can be a subjective trait (for on the species . The fusion of diploid gametes leads to homologous chromosomes in allopolyploids prevents example, age of reproduction), tetraploid zygotes, which are potentially stable. intergenomic recombination, effectively maintaining a more precise definition is There is a basic distinction between autopolyploids the same level of heterozygosity through the genera- non-additive inheritance in (FIGS 2,3) which a trait in the F1 and allopolyploids. Both have multiple sets of chromo- tions
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