Why are allopolyploid sterile

To verify their hybrid nature, we subjected randomly selected hybrids and the parental strains to electrophoretic karyotyping. As shown in Fig. Electrophoretic karyotypes of the parental strains and four synthetic hybrids. Chromosome numbering is shown on the left size for S. The numbering is based on the reference Antunovics et al. Each hybrid was tested for sporulation by culturing on sporulation medium and for spore viability by random spore analysis Table 3.

Segregation of auxotrophic markers in random spore analysis was taken as evidence of production of viable spores. The rest of the hybrids produced more spores and could thus be subjected to random spore analysis. None of them could conjugate with the mating type testers listed in Table 1. Seventy per cent of the hybrids were not sterile because they produced auxotrophic segregants. We hereafter call the viable spores of the hybrids F1 spores and refer to the clones of vegetative cells produced by them as F1 clones.

Hybrids and their offspring analysed in detail. Tetrads and spore clones with black background were used for production of filial generations. Percentages of viable spores are shown in brackets. Sporulation of hybrids and segregation of auxotrophic markers in random spore analysis. However, the spores of the H21 and H29 F1 clones were all dead. Thus, these hybrids were F1-sterile.

In contrast, certain F1 clones of the hybrids H10 and H35 produced viable F2 spores, indicating that these hybrids were not fully F1-sterile. From one of the F2 spore clones, we isolated tetrads of F3 spores and found high levels of spore viability. Then, we isolated spores from two F3 clones to produce F4 generation. The F4 clones obtained also sporulated efficiently.

The genealogy of the spore clones and the spore viability in selected clones are shown in Fig. To explore the genomes of the hybrids and their descendents, we tested the spore clones for the presence of parental markers and chromosomes.

The eleven markers covered eight chromosomes of the S. As no 1 : 3 and 3 : 1 tetrads were found, we presumed that the wild-type S. Both categories were tested for mating activity with S. In brackets: the chromosomal location of the gene in the Saccharomyces cerevisiae genome.

Fragment size. The four hybrids analysed were heterozygous for all molecular markers Table 4. Their descendents also possessed both parental orthologues of most genes. In a few spore clones, the S. These results confirmed that the entire Chr. The copy number of the leu2 allele can be higher in the hybrid when the S.

III of the S. To verify the absence of a S. In our previous study on a similar synthetic hybrid Antunovics et al. In those clones, the S. Remarkably, the HIS4 orthologue is also located on scaffold s10 in the S.

To ascertain whether this scaffold corresponded to the band that was missing in the karyotypes of the fertile F1 clones, we amplified the S.

As presumed, the probe reacted with Chr. Probing the karyotypes with a labelled S. The only exception was Chr. III which does not bind the probe Antunovics et al. However, certain spore clones differed in signal intensity at the poorly resolved group of the largest chromosomal bands Fig. By modifying the running parameters of electrophoresis, we managed to separate the bands and found no segregation Fig.

Chromosomal patterns and hybridisation with Saccharomyces cerevisiae -specific subtelomeric probe in hybrids and F1 descendents. One example of tetrads is shown for each hybrid.

Arrowheads mark Saccharomyces uvarum Chr. Hybridisation of labelled LEU2 to chromosomal bands. Lane numbering is as for Fig. Arrowhead with double line: Saccharomyces uvarum Chr. Arrowhead with single line: Saccharomyces cerevisiae Chr. Separation of large chromosomes and hybridization with Saccharomyces cerevisiae -specific subtelomeric probe. Numbering of S. To reveal further differences between the genomes of the hybrids and their descendants, we performed RAPD analysis with two primers and amplified interdelta sequences Table 4.

There were only two S. The amplification of interdelta sequences revealed higher diversity. No fragments were amplified from the S. The inability of the S. As Chr. To test whether a similar interaction may also take place between the MAT loci of the hybridised species, we cloned the MAT cassettes of the S. We obtained both conjugating and nonconjugating transformants. The sterility of the nonconjugating transformants was unstable. The sterility of the transformants and the simultaneous loss of their prototrophy and sterility demonstrated that a S.

Using the mass-mating method based on complementation of auxotrophic markers, we generated 57 novel synthetic hybrids of S. As the mating mixture contained both spores and vegetative cells, interspecies hybrids could arise from all possible combinations of spores and vegetative cells of the partners.

These combinations were unlikely to produce hybrids of identical ploidy because spores have smaller genomes than vegetative cells.

Consistent with this, the hybrids displayed differences in sporulation efficiency, spore viability and marker segregation. Seventeen hybrids did not form viable spores. This property is reminiscent of the sterility of interspecies hybrids of higher organisms.

Thus, these S. This conclusion is consistent with our earlier observation that hybrids formed by S. The rest of the hybrids were not sterile because they produced spores that germinated and produced colonies. The viability of their F1 spores indicated that they had at least two sets of chromosomes from both progenitor species allotetraploid genomes.

The viability of ascospores does not necessarily mean that the hybrid is fertile. Homothallic haploid cells are able to convert their mating type almost as frequently as every cell division, so conjugation can take place between vegetative descendants of a homothallic spore.

As heterozygosity at the MAT locus represses the genes of the conjugation pathway Nasmyth, , these spores and their vegetative descendents should not be able to conjugate. However, if the hybrid is allotetraploid, its F1 spores are very likely allodiploid and unable to produce viable F2 spores.

On the basis of these considerations, we presumed sterility in the F1 clones of our hybrids. Consistent with this, two of the four hybrids subjected to tetrad analysis did not form viable F2 spores.

In the other two hybrids analysed, many but not all F1 clones produced viable F2 descendants, demonstrating that F1 sterility can be broken down. When comparing the genotypes and fertility of the spore clones, we found perfect correlation between the loss of F1 sterility, the loss of the S.

This coincidence suggests that the abolishment of F1 sterility is attributable to the loss of Chr. How can the elimination of a chromosome from the genome make the alloploid hybrid fertile? As the S. We believe that this may account for the observed gain of fertility in the leucine auxotrophic alloploid F1 clones.

Thus, the F1 sterility of the allotetraploid S. Being nullisomic for the S. III in the S. These nullisomes behave in sex determination as if they were haploid. They can switch their mating-type in haploid-like manner and can also activate the conjugation programme. The zygotes produced by the conjugating nullisomes of opposite mating types are also nullisomic for Chr.

This heterozygosity prevents them from another mating and enables them to perform meiosis leading to viable alloaneuoploid F2 ascospores nullisomic for Chr. A model of breaking F1 sterility by chromosome loss. A MAT -carrying chromosome can be lost either during meiosis as shown or prior to it, during vegetative propagation of the hybrid cells.

A crucial element of the model is the assumption that the MAT -encoded regulators of the partner species interact in the hybrid cells essentially in the same way as they do in their own cells and this interaction makes the interspecies hybrids sterile or F1 sterile.

The assumed regulatory interaction needs experimental verification, but the loss of mating activity in the S. The phenotypic and molecular characterization of the spore clones revealed several changes in the F1 to F4 genomes compared to the hybrid genome. Most changes were directional, affecting mainly the S. Besides loosing of Chr.

Interestingly, the loss of these genes was not associated with karyotype alterations, indicating that mechanisms different from aneuploidization can also cause directional changes.

These mechanisms may involve intergenomic nonhomoeologous, homoeologous recombination, such as conversion of the S. Much fewer directional changes were detected in the S. As delta sequences were present only in the S.

Delta sequences are associated with mobile genetic elements and changes in their location indicate chromosomal rearrangements Argueso et al. Transposon activity has been recently implicated in genomic changes in synthetic Arabidopsis polyploids Madlung et al. Preferential elimination of genomic loci of one of the hybridizing partners was observed in our previous work in a similar yeast hybrid Antunovics et al. In wheat, DNA sequences that are genome and chromosome specific in established allopolyploids are deleted at high frequency from parental genomes in synthetic allopolyploids Feldman et al.

In synthetic hybrids of Brassica nigra and Brassica rapa , the former genome lost more genomic fragments Song et al. Using a large number of synthetic alloploid hybrids, we have demonstrated that the nascent S.

The two types of infertility indicate that the postzygotic reproductive isolation of these species is ensured by double sterility barrier: by hybrid sterility hybrid cells cannot produce viable spores operating in allodiploids and by F1 sterility F1 cells cannot produce viable spores operating in allopolyploids.

Hybrid sterility has been found to be due to the inability of homoeologous chromosomes to pair recombine in meiosis Greig, Here, we show that F1-sterility is ascribable to mating-type heterozygosity and can be circumvented by elimination of Chr. To the best of our knowledge, this is the first report on breaking down interspecies hybrid sterility by chromosome loss. Phenotypic instability and rapid gene silencing in newly formed Arabidopsis allotetraploids. Plant Cell 12 , Fransz, P. Interphase chromosomes in Arabidopsis are organized as well defined chromocenters from which euchromatin loops emanate.

Proceedings of the National Academy of Sciences 99 , Guo, M. Dosage effects on gene expression in a maize ploidy series.

Genetics , Melaragno, J. Relationship between endopolyploidy and cell size in epidermal tissue of Arabidopsis. Plant Cell 5 , Mittelsten Scheid, O.

A change of ploidy can modify epigenetic silencing. Proceedings of the National Academy of Sciences 93 , Shaked, H. Sequence elimination and cytosine methylation are rapid and reproducible responses of the genome to wide hybridization and allopolyploidy in wheat. Plant Cell 13 , Van de Peer, Y. Chapter 6: Large-scale gene and ancient genome duplications.

In The Evolution of the Genome , ed. Gregory, San Diego, Elsevier, Wang, J. Genome-wide nonadditive gene regulation in Arabidopsis allotetraploids.

Chromosome Mapping: Idiograms. Human Chromosome Translocations and Cancer. Karyotyping for Chromosomal Abnormalities. Prenatal Screen Detects Fetal Abnormalities. Synteny: Inferring Ancestral Genomes. Telomeres of Human Chromosomes. Chromosomal Abnormalities: Aneuploidies. Chromosome Abnormalities and Cancer Cytogenetics. Copy Number Variation and Human Disease. Genetic Recombination. Human Chromosome Number.

Trisomy 21 Causes Down Syndrome. X Chromosome: X Inactivation. Chromosome Theory and the Castle and Morgan Debate. Developing the Chromosome Theory. Meiosis, Genetic Recombination, and Sexual Reproduction. Mitosis and Cell Division. Genetic Mechanisms of Sex Determination. Sex Chromosomes and Sex Determination. Sex Chromosomes in Mammals: X Inactivation. Sex Determination in Honeybees.

Polyploidy By: Margaret Woodhouse, Ph. Citation: Woodhouse, M. Nature Education 2 1 How does this interesting condition crop up, and what advantages and disadvantages does it impart? Aa Aa Aa. Mechanisms of Polyploidy. Figure 1. Figure Detail. Figure 2. Advantages of Polyploidy. Figure 3: Polyploid formation and ensuing meiotic and mitotic irregularities. The figure illustrates the chromosomal composition and behavior of diploids and derived polyploids at different developmental times in meiosis a, b and mitosis c.

The advantages and disadvantages of being polyploid. Nature Reviews Genetics 6, All rights reserved. Disadvantages of Polyploidy. Evolutionary Potential of Polyploid Organisms. Nature Reviews Genetics 6 , link to article Adams, K. Plant Cell 12 , Fransz, P. Genetics , Melaragno, J.

When one of the resulting gametes 2n combines with a regular haploid n gamete, the resulting offspring are triploid 3n. Offspring produced in this way are normally infertile because they have an uneven number of chromosomes that won't pair correctly during meiosis.

When two of these gametes 2n combine, the resulting offspring are tetraploid 4n. This is common in plants as they produce both male and female gametes and are often capable of self-fertilisation. The resulting offspring a generally fertile as they have an even number of chromosomes. If the new tetraploid offspring cannot reproduce with the parental type diploid plants, but can reproduce with each other, a new species has been formed.

The resulting hybrid is usually sterile because the chromosomes from each species cannot pair correctly during meiosis.

The two different species involved may also contribute different numbers of chromosomes which again prevents chromosome pairing during meiosis, rendering the hybrid sterile. This process is known as Amphipolyploidy.

Allopolyploidy generally produces infertile hybrids because the chromosomes from each of the parent species cannot pair correctly.