Polyploid and Hybrid Genomics

Edited by
Z. JEFFREY CHEN (The University of Texas at Austin)

and
JAMES A. BIRCHLER (University of Missouri Columbia)

This edition first published 2013 © 2013 by John Wiley & Sons, Inc.

1. Yeast Hybrids and Polyploids as Models in Evolutionary Studies

Avraham A. Levy¹, Itay Tirosh², Sharon Reikhav^1,2, Yasmin Bloch^1,2, and Naama Barkai²

1Department of Plant Sciences, The Weizmann Institute of Science, Rehovot, Israel
2Department of Molecular Genetics, The Weizmann Institute of Science, Rehovot, Israel

Yeast Hybrids

        Several explanations for this ubiquitous(无处不在的) hybridity have been proposed, such as the potential phenotypic advantages of the hybrids (e.g., heterosis), their utilization in breeding yeast strains (Timberlake et al., 2011), their ability to survive following speciation, due to asexual reproduction, and to become, in the long-term, stabilized as distinct species through genomic rearrangements (Antunovics et al., 2005), or through genome doubling (Naumov et al., 2000b). All these show that most hybrid-specific phenomena reported in higher eukaryotes are also present in the yeast system. Hence, this system is highly suitable to model hybridity and polyploidy also in higher plants and other organisms.

对于这种普遍存在的hybridity现象，人们提出了几种解释，比如可能性 杂交种的表型优势(如杂种优势)及其在酵母菌育种中的应用(Timberlake et al.， 2011)，它们在物种形成后的生存能力，由于无性繁殖， 并在长期内，通过基因组重排稳定成为不同的物种(Antunovics et al.， 2005)，或通过基因组加倍(Naumov et al.， 2000b)。所有这些都表明 在高等真核生物中报道的大多数杂交种特异性现象也存在于酵母系统中。因此，该系统非常适合于高等植物和其他生物的杂交和多倍体模式。

         Hybrid incompatibility genes, also called “speciation genes” as originally described in the 1930s(Dobzhansky, 1936), were isolated in several species (Johnson, 2010; Presgraves, 2010). However,the search for such genes in budding yeasts has been unsuccessful despite the significant efforts invested (Greig, 2007, 2009). The lack of incompatibility genes explains why closely related species of budding yeast mate readily and usually with no major deleterious interactions, except for the hybrid’s sterility (Hunter et al., 1996; Marinoni et al., 1999). 

杂交不亲和性基因，也被称为“物种形成基因”，最初描述于20世纪30年代(Dobzhansky, 1936)，在几个物种中被分离出来(Johnson, 2010;来自,2010)。然而，尽管投入了大量的努力，在芽殖酵母中寻找这类基因仍然不成功(Greig, 2007, 2009)。缺乏不亲和性基因解释了为什么近缘种的出芽酵母容易交配，通常没有主要的有害相互作用，除了杂交种的不育性(Hunter et al.， 1996;Marinoni等人，1999年)。

This sterility is probably caused by defective pairing of divergent chromosomes at meiosis rather than by the role of specific speciation genes. It does not prevent the vegetative propagation of the sterile hybrid; however, it may limit its long-term prospects for survival. Speciation may thus have occurred through physical rather than genetic isolation, although this possibility is not supported by the frequent occurrence of hybrids alongside their parental species (Le Jeune et al., 2007). Note that the sterility of diploid hybrids (homoploids) can be overcome upon genome duplication, giving rise to allopolyploids (also known as amphiploids) that are fertile, with most of the spores being viable (Greig et al., 2002).

这种不育性可能是由减数分裂时染色体的缺陷配对造成的，而不是由特定的物种形成基因所起的作用。它并不妨碍不育杂交种的营养繁殖;然而，这可能会限制其长期生存的前景。因此，物种形成可能是通过物理隔离而不是遗传隔离发生的，尽管这种可能性并没有被频繁出现的与亲本物种杂交所支持(Le Jeune et al.， 2007)。值得注意的是，二倍体杂种(同倍体)的不育性可以通过基因组复制来克服，从而产生可育的异源多倍体(也称为二倍体)，其中大多数孢子是有活力的(Greig et al.， 2002)。

       Not surprisingly, considering their success in nature and under domestication, yeast interspecific

hybrids were reported to show heterosis (Tirosh et al., 2009). The genetic and molecular basis of

heterosis in yeast has received very little attention so far despite its importance for the yeast industry and its potential utility as a model for understanding heterosis in plants and animal breeding. Among the few reports, quantitative trait locus (QTL) mapping of genes involved in yeast growth under high temperatures uncovered a complex locus of three genes, which when heterozygous contributed to heterosis (Steinmetz et al., 2002).

        毫不奇怪，考虑到它们在自然界的成功和驯化，据报道酵母菌种间杂交具有杂种优势(Tirosh et al.， 2009)。遗传和分子基础 尽管酵母杂种优势在酵母工业中的重要性及其作为植物和动物育种中理解杂种优势的模型的潜在用途，但迄今为止酵母杂种优势的研究还很少受到关注。在为数不多的报道中，对高温下与酵母生长有关的基因进行定量性状位点(QTL)定位发现了一个当杂合子导致杂种优势时由三个基因组成的复杂位点(Steinmetz et al.， 2002)。

 Good words and sentences 

          Several studies showed that species from the Saccharomyces genus are prone to(倾向于) interspecific hybridization, either naturally or through domestication in breweries and wineries and in laboratories.(see review, Albertin & Marullo, 2012).

Interestingly, the speciation process that gives rise to new yeast species is not well understood.

形成酵母新物种的物种形成过程

** Professor Avraham A. Levy**

Weizmann Institute of Science
P.O.B. 26 Rehovot 76100, Israel
Telephone: (+972-8) 9342734
Fax: (+972-8) 934-4181

avi.levy@weizmann.ac.ilCV

研究兴趣（from http://www.weizmann.ac.il/plants/levy/）：

The goal of our research is to understand the mechanisms that determine the plasticity and evolution of the plant genome. Plants are fast runners in the evolutionary race. There are hundreds of thousands of plant species, genome size is highly variable in the plant kingdom, some species have ~100,000 genes (4 times more than Human) and plants show high tolerance to genetic alterations, including to changes in ploidy.  In a single species such as maize the amount of genetic diversity is greater than in all primates. We are studying the contribution of mechanisms such as transposition, DNA recombination (homologous and non-homologous), hybridization and poyploidy, to rapid genome evolution.   A general theme of our research is the connection between epigenetic modifications and the maintenance of genome integrity.  In addition, we are harnessing DNA repair mechanisms for developing new approaches for precise genome editing for plant breeding.  Our favorite organisms are Arabidopsis, tomato, wheat and yeast.