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Reproductive Isolation in the Elegans-Group of Caenorhabditis

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Reproductive Isolation in the Elegans-Group of Caenorhabditis Wright State University CORE Scholar Biological Sciences Faculty Publications Biological Sciences 3-7-2013 Reproductive Isolation in the Elegans-Group of Caenorhabditis Scott Everet Baird Wright State University - Main Campus, [email protected] Sara Rose Seibert Wright State University - Main Campus Follow this and additional works at: https://corescholar.libraries.wright.edu/biology Part of the Biology Commons, Evolution Commons, Genetics Commons, Medical Sciences Commons, and the Systems Biology Commons Repository Citation Baird, S. E., & Seibert, S. R. (2013). Reproductive Isolation in the Elegans-Group of Caenorhabditis. Natural Science, 5 (4A), 18-25. https://corescholar.libraries.wright.edu/biology/144 This Article is brought to you for free and open access by the Biological Sciences at CORE Scholar. It has been accepted for inclusion in Biological Sciences Faculty Publications by an authorized administrator of CORE Scholar. For more information, please contact [email protected]. Vol.5, No.4A, 18-25 (2013) Natural Science http://dx.doi.org/10.4236/ns.2013.54A004 Reproductive isolation in the Elegans-Group of Caenorhabditis Scott Everet Baird*, Sara R. Seibert Department of Biological Sciences, Wright State University, Dayton, USA; *Corresponding Author: [email protected] Received 21 January 2013; revised 20 February 2013; accepted 7 March 2013 Copyright © 2013 Scott Everet Baird, Sara R. Seibert. This is an open access article distributed under the Creative Commons Attri- bution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ABSTRACT and seasonality of reproduction. Assortative fertilization refers to mechanisms that prevent fertilization after mat- Reproductive isolation is the basis of the Bio- ing has occurred. These mechanisms include species-spe- logical Species Definition and can be a driving cific differences in sperm chemotaxis and sperm-ova force of speciation. Theoretical studies have pro- fusion. Post-zygotic isolation refers to mechanisms that vided models of how reproductive isolation can decrease the fitness of hybrids relative to that of the pa- arise within individual species. Genetic tests of rental populations. These mechanisms include hybrid le- these models are limited to populations in which thality, hybrid sterility and hybrid breakdown. reproductive isolation is present but not com- Early speciation models required the origin reproduc- plete. Here, reproductive isolation in the Elgans- tive isolation in allopatric populations [1]. In this way, Group of the nematode genus Caenorhabditis is dysgenic combinations of alleles would not be subjected reviewed. Pre-mating barriers, assortative fertili- to negative selection before they became fixed. More zation and post-zygotic barriers all have been recently, models of speciation with gene flow have been observed in this clade. In some combinations of developed [8,9,12]. In such models, reproductive isola- species, fertile F1 hybrids can be obtained. There- tion does not act uniformly across the entire genome [6]. fore, the Elegans-Group of Caenorhabditis is Rather, gene flow will be restricted only in regions link- poised to become an important experimental ed to loci involved in reproductive isolation [6,9,12]. system for the study of reproductive isolation. To gain a broad understanding of speciation, empirical studies in diverse taxa are required. To date, nematodes Keywords: Caenorhabditis; Pre-Mating Isolation; in general, and Caenorhabditis in particular, have con- Assortative Fertilization; Post-Zygotic Isolation tributed little to these studies [13-20]. However, in recent years, a number of new Caenorhabditis species have been 1. INTRODUCTION discovered [18,21]. With these new species, four pair- wise combinations that yield fertile F1 hybrids have been Reproductive isolation refers collectively to all genetic described [17-19]. With these new resources, Caenor- mechanisms that prevent or limit gene flow between habditis is poised to become an important speciation ge- populations [1]. These mechanisms are thought to result netics model system. This review provides an overview from allele-specific dysgenic interactions between two or of reproductive isolation in the Elegans-Group of Caenor- more genes [2-5]. Reproductive isolation is the basis of habditis. the Biological Species Definition [1]. As reproductive isolation restricts gene flow, it also can facilitate genomic 2. THE ELEGANS-GROUP divergence between populations [6-9]. Categories of re- productive isolating mechanisms include pre-mating iso- Caenorhabditis is a genus of microbiophagus nema- lation, assortative fertilization and post-zygotic isolation todes that reproduce on rotting fruit [21,22]. Caenorhab- [1,5,10,11]. Pre-mating isolation refers to mechanisms ditis species reproduce rapidly with generation times of that prevent matings between individuals from different three to four days at 20˚C. Like other nematodes, Caenor- species. These mechanisms include species-specific dif- habditis species form developmentally arrested, alterna- ferences in reproductive structures, behavior, ecology, tive third stage, dauer larvae in conditions of environ- Copyright © 2013 SciRes. OPEN ACCESS S. E. Baird, S. R. Seibert / Natural Science 5 (2013) 18-25 19 mental stress [23]. Most if not all species of Caenorhab- are known only from geographically limited collection ditis form phoretic associations with other soil inverte- sites [21]. However, at least four, C. elegans, C. briggsae, brates as dauers [24]. These associations are used for C. remanei and C. brenneri have cosmopolitan distribu- transport to microenvironments conducive to reproduc- tions [21,37]. Of these, populations are most highly struc- tion. tured in C. briggsae [38]. One member of Caenorhabditis, C. elegans is a model Many pairwise combinations of Elegans-Group spe- organism that has been used extensively in genetic and cies are cross-fertile (Figure 2). In most cross-fertile genomic studies [e.g. 25-27]. More recently, additional combinations, F1 hybrids arrest during embryogenesis members of Caenorhabditis have contributed to a variety [13,14,21]. In six combinations, F1 adults are obtained of evolutionary studies [28-30]. Mostly, other the species (Table 1) [13,16-18]. In four of these combinations, fer- of Caenorhabditis used in these studies have been from tile F1 adults are obtained [17,18]. These results demon- the Elegans-Group. strate that the Elegans-Group of Caenorhabditis affords The Elegans-Group is a monophyletic clade within ample opportunities for genetic and genomic studies of Caenorhabditis (Figure 1). Live cultures of ten Elegans- reproductive isolation. Group species are available. C. elegans is the basal mem- ber or this group [21]. The development of this species 3. REPRODUCTIVE ISOLATION IN THE has been characterized extensively [31,32]. All species ELEGANS-GROUP within the Elegans-Group are morphologically similar and follow similar patterns of development [14,33]. The 3.1. Pre-Mating Isolation genomes of several Elegans-Group Species have been Within the Elegans-Group of Caenorhabditis, all pair- sequenced or are in the process of being sequenced and wise combinations of species that have been tested will assembled [34-36]. Curated lists of orthologous genes mate with each other [13,17,18,21]. Therefore, premat- can be found at wormbase.org. ing mechanisms do not obviously contribute to reproduc- Most species in the Elegans-Group of Caenorhabditis tive isolation. This conclusion has two major caveats. The first caveat is that all mating tests between Cae- norhabditis species were conducted on agar plates with E. coli as a food source. In natural populations, Caenorhab- ditis species reproduce on rotting fruit while feeding on a diverse microbial community [21,22]. Pre-mating mecha- nisms not apparent in laboratory conditions may act in natural populations. For example, C. elegans and C. brig- gsae both reproduce on rotting fruit and have been found to co-occur on the same fruit [22]. These species appear Figure 1. Phylogenetic relationships within the Elegans-Group of Cae- norhabditis [18,21]. C.oncomelaniae, C. formosana and C. clavopapillata are included within the Elegans- Group based on morphological cha- racters [37]. live cultures of these three species currently are not avai- lable, which precludes their inclusion in phylogenies, such as this one, that are based on DNA sequence compa- Figure 2. Cross fertility among Elegans-Group species [13-18, risions. 21,57]. Copyright © 2013 SciRes. OPEN ACCESS 20 S. E. Baird, S. R. Seibert / Natural Science 5 (2013) 18-25 Table 1. Hybrid sterility and hybrid breakdown in the Elegans-Group. males females/herm.a outcome ref. fertile F1 females, slow developing sterile F1 males, C. briggsae C. sp. 9 fertile F2 females and males obtained from crosses of [17,19] C. sp. 9 males to F1 females fertile F1 females, no adult F1 males, fertile F2 C. sp. 9 C. briggase females and males obtained from crosses of C. sp. 9 [17,19] males to F1 females C. briggsae C. sp. 5 sterile F1 females with abnormal gonads [21] sterile F1 females with abnormal gonads, XO hybrids C. briggsae C. remanei develop as males, intersexes or females, degree of [13,16] sexual transformation strain-dependent fertile F1 females and fertile F1 males, C. remanei C. sp. 23 [18] F2 hybrid breakdown (embryonic lethality) fertile F1 females and fertile F1 males, C. sp. 23 C. remanei [18] F2
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