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Soto TY, Rojas-Gutierrez JD, Oakley CG. Can heterosis and inbreeding depression explain the maintenance of outcrossing in a cleistogamous perennial? AMERICAN JOURNAL OF BOTANY 2023; 110:e16240. [PMID: 37672596 DOI: 10.1002/ajb2.16240] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/09/2023] [Revised: 08/17/2023] [Accepted: 08/18/2023] [Indexed: 09/08/2023]
Abstract
PREMISE What maintains mixed mating is an evolutionary enigma. Cleistogamy-the production of both potentially outcrossing chasmogamous and obligately selfing cleistogamous flowers on the same individual plant-is an excellent system to study the costs of selfing. Inbreeding depression can prevent the evolution of greater selfing within populations, and heterosis in crosses between populations may further tip the balance in favor of outcrossing. Few empirical estimates of inbreeding depression and heterosis in the same system exist for cleistogamous species. METHODS We investigate the potential costs of selfing by quantifying inbreeding depression and heterosis in three populations of the cleistogamous perennial Ruellia humilis Nutt (Acanthaceae). We performed three types of hand-pollinations-self, outcross-within, and outcross-between populations-and measured seed number, germination, total flower production, and estimated cumulative fitness for the resulting progeny in a greenhouse experiment. RESULTS We found moderate inbreeding depression for cumulative fitness (<30%) in two populations, but outbreeding depression for crosses within a third population (-26%). For between-population crosses, there was weak to modest heterosis (11-47%) in two of the population combinations, but modest to strong outbreeding depression (-21 to -71%) in the other four combinations. CONCLUSIONS Neither inbreeding depression nor heterosis was of sufficient magnitude to explain the continued production of chasmogamous flowers given the relative energetic advantage of cleistogamous flowers previously estimated for these populations. Outbreeding depression either within or between populations makes the maintenance of chasmogamous flowers even harder to explain. More information is needed on the genetic basis of cleistogamy to resolve this conundrum.
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Affiliation(s)
- Tatyana Y Soto
- Department of Botany and Plant Pathology and the Center for Plant Biology, Purdue University, West Lafayette, IN, USA
| | - Juan Diego Rojas-Gutierrez
- Department of Botany and Plant Pathology and the Center for Plant Biology, Purdue University, West Lafayette, IN, USA
| | - Christopher G Oakley
- Department of Botany and Plant Pathology and the Center for Plant Biology, Purdue University, West Lafayette, IN, USA
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Hudson CM, Cuenca Cambronero M, Moosmann M, Narwani A, Spaak P, Seehausen O, Matthews B. Environmentally independent selection for hybrids between divergent freshwater stickleback lineages in semi-natural ponds. J Evol Biol 2023; 36:1166-1184. [PMID: 37394735 DOI: 10.1111/jeb.14194] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/16/2022] [Revised: 05/03/2023] [Accepted: 05/31/2023] [Indexed: 07/04/2023]
Abstract
Hybridization following secondary contact of genetically divergent populations can influence the range expansion of invasive species, though specific outcomes depend on the environmental dependence of hybrid fitness. Here, using two genetically and ecologically divergent threespine stickleback lineages that differ in their history of freshwater colonization, we estimate fitness variation of parental lineages and hybrids in semi-natural freshwater ponds with contrasting histories of nutrient loading. In our experiment, we found that fish from the older freshwater lineage (Lake Geneva) and hybrids outperformed fish from the younger freshwater lineage (Lake Constance) in terms of both growth and survival, regardless of the environmental context of our ponds. Across all ponds, hybrids exhibited the highest survival. Although wild-caught adult populations differed in their functional and defence morphology, it is unclear which of these traits underlie the fitness differences observed among juveniles in our experiment. Overall, our work suggests that when hybrid fitness is insensitive to environmental conditions, as observed here, introgression may promote population expansion into unoccupied habitats and accelerate invasion success.
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Affiliation(s)
- Cameron Marshall Hudson
- Department of Fish Ecology and Evolution, Eawag, Swiss Federal Institute of Aquatic Science and Technology, Center of Ecology, Evolution and Biochemistry, Lucerne, Switzerland
- Division of Aquatic Ecology and Evolution, Institute of Ecology and Evolution, University of Bern, Bern, Switzerland
- Department of Aquatic Ecology, Eawag, Swiss Federal Institute of Aquatic Science and Technology, Zürich, Switzerland
| | - Maria Cuenca Cambronero
- Department of Fish Ecology and Evolution, Eawag, Swiss Federal Institute of Aquatic Science and Technology, Center of Ecology, Evolution and Biochemistry, Lucerne, Switzerland
- Aquatic Ecology Group, University of Vic, Central University of Catalonia, Vic, Spain
| | - Marvin Moosmann
- Department of Fish Ecology and Evolution, Eawag, Swiss Federal Institute of Aquatic Science and Technology, Center of Ecology, Evolution and Biochemistry, Lucerne, Switzerland
- Division of Aquatic Ecology and Evolution, Institute of Ecology and Evolution, University of Bern, Bern, Switzerland
| | - Anita Narwani
- Department of Aquatic Ecology, Eawag, Swiss Federal Institute of Aquatic Science and Technology, Zürich, Switzerland
| | - Piet Spaak
- Department of Aquatic Ecology, Eawag, Swiss Federal Institute of Aquatic Science and Technology, Zürich, Switzerland
| | - Ole Seehausen
- Department of Fish Ecology and Evolution, Eawag, Swiss Federal Institute of Aquatic Science and Technology, Center of Ecology, Evolution and Biochemistry, Lucerne, Switzerland
- Division of Aquatic Ecology and Evolution, Institute of Ecology and Evolution, University of Bern, Bern, Switzerland
| | - Blake Matthews
- Department of Fish Ecology and Evolution, Eawag, Swiss Federal Institute of Aquatic Science and Technology, Center of Ecology, Evolution and Biochemistry, Lucerne, Switzerland
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Mating system and speciation I: Accumulation of genetic incompatibilities in allopatry. PLoS Genet 2022; 18:e1010353. [PMID: 36520924 PMCID: PMC9799327 DOI: 10.1371/journal.pgen.1010353] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/22/2022] [Revised: 12/29/2022] [Accepted: 11/25/2022] [Indexed: 12/23/2022] Open
Abstract
Self-fertilisation is widespread among hermaphroditic species across the tree of life. Selfing has many consequences on the genetic diversity and the evolutionary dynamics of populations, which may in turn affect macroevolutionary processes such as speciation. On the one hand, because selfing increases genetic drift and reduces migration rate among populations, it may be expected to promote speciation. On the other hand, because selfing reduces the efficacy of selection, it may be expected to hamper ecological speciation. To better understand under which conditions and in which direction selfing affects the build-up of reproductive isolation, an explicit population genetics model is required. Here, we focus on the interplay between genetic drift, selection and genetic linkage by studying speciation without gene flow. We test how fast populations with different rates of selfing accumulate mutations leading to genetic incompatibilities. When speciation requires populations to pass through a fitness valley caused by underdominant and compensatory mutations, selfing reduces the depth and/or breadth of the valley, and thus overall facilitates the fixation of incompatibilities. When speciation does not require populations to pass through a fitness valley, as for Bateson-Dobzhanzky-Muller incompatibilities (BDMi), the lower effective population size and higher genetic linkage in selfing populations both facilitate the fixation of incompatibilities. Interestingly, and contrary to intuitive expectations, local adaptation does not always accelerate the fixation of incompatibilities in outcrossing relative to selfing populations. Our work helps to clarify how incompatibilities accumulate in selfing vs. outcrossing lineages, and has repercussions on the pace of speciation as well as on the genetic architecture of reproductive isolation.
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Clo J, Ronfort J, Gay L. Fitness consequences of hybridization in a predominantly selfing species: insights into the role of dominance and epistatic incompatibilities. Heredity (Edinb) 2021; 127:393-400. [PMID: 34365470 PMCID: PMC8478955 DOI: 10.1038/s41437-021-00465-2] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/15/2021] [Revised: 07/29/2021] [Accepted: 07/30/2021] [Indexed: 02/07/2023] Open
Abstract
Studying the consequences of hybridization on plant performance is insightful to understand the adaptive potential of populations, notably at local scales. Due to reduced effective recombination, predominantly selfing species are organized in highly homozygous multi-locus-genotypes (or lines) that accumulate genetic differentiation both among- and within-populations. This high level of homozygosity facilitates the dissection of the genetic basis of hybrid performance in highly selfing species, which gives insights into the mechanisms of reproductive isolation between lines. Here, we explored the fitness consequences of hybridization events between natural inbred lines of the predominantly selfing species Medicago truncatula, at both within- and among-populations scales. We found that hybridization has opposite effects pending on studied fitness proxies, with dry mass showing heterosis, and seed production showing outbreeding depression. Although we found significant patterns of heterosis and outbreeding depression, they did not differ significantly for within- compared to among-population crosses. Family-based analyses allowed us to determine that hybrid differentiation was mostly due to dominance and epistasis. Dominance and/or dominant epistatic interactions increased dry mass, while decreasing seed production, and recessive epistatic interactions mostly had a positive effect on both fitness proxies. Our results illustrate how genetic incompatibilities can accumulate at a very local scale among multi-locus-genotypes, and how non-additive genetic effects contribute to heterosis and outbreeding depression.
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Affiliation(s)
- Josselin Clo
- grid.463758.b0000 0004 0445 8705AGAP, Univ Montpellier, CIRAD, INRAE, Institut Agro, Montpellier, France ,grid.4491.80000 0004 1937 116XDepartment of Botany, Faculty of Science, Charles University in Prague, Prague, Czech Republic
| | - Joëlle Ronfort
- grid.463758.b0000 0004 0445 8705AGAP, Univ Montpellier, CIRAD, INRAE, Institut Agro, Montpellier, France
| | - Laurène Gay
- grid.463758.b0000 0004 0445 8705AGAP, Univ Montpellier, CIRAD, INRAE, Institut Agro, Montpellier, France
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Waller DM. Addressing Darwin's dilemma: Can pseudo-overdominance explain persistent inbreeding depression and load? Evolution 2021; 75:779-793. [PMID: 33598971 DOI: 10.1111/evo.14189] [Citation(s) in RCA: 17] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/11/2020] [Revised: 01/06/2021] [Accepted: 01/30/2021] [Indexed: 01/01/2023]
Abstract
Darwin spent years investigating the effects of self-fertilization, concluding that "nature abhors perpetual self-fertilization." Given that selection purges inbred populations of strongly deleterious mutations and drift fixes mild mutations, why does inbreeding depression (ID) persist in highly inbred taxa and why do no purely selfing taxa exist? Background selection, associations and interference among loci, and drift within small inbred populations all limit selection while often increasing fixation. These mechanisms help to explain why more inbred populations in most species consistently show more fixed load. This drift load is manifest in the considerable heterosis regularly observed in between-population crosses. Such heterosis results in subsequent high ID, suggesting a mechanism by which small populations could retain variation and inbreeding load. Multiple deleterious recessive mutations linked in repulsion generate pseudo-overdominance. Many tightly linked load loci could generate a balanced segregating load high enough to sustain ID over many generations. Such pseudo-overdominance blocks (or "PODs") are more likely to occur in regions of low recombination. They should also result in clear genetic signatures including genomic hotspots of heterozygosity; distinct haplotypes supporting alleles at intermediate frequency; and high linkage disequilibrium in and around POD regions. Simulation and empirical studies tend to support these predictions. Additional simulations and comparative genomic analyses should explore POD dynamics in greater detail to resolve whether PODs exist in sufficient strength and number to account for why ID and load persist within inbred lineages.
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Affiliation(s)
- Donald M Waller
- Department of Botany, University of Wisconsin-Madison, Madison, Wisconsin, 53706
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Pickup M, Brandvain Y, Fraïsse C, Yakimowski S, Barton NH, Dixit T, Lexer C, Cereghetti E, Field DL. Mating system variation in hybrid zones: facilitation, barriers and asymmetries to gene flow. THE NEW PHYTOLOGIST 2019; 224:1035-1047. [PMID: 31505037 PMCID: PMC6856794 DOI: 10.1111/nph.16180] [Citation(s) in RCA: 20] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/14/2019] [Accepted: 08/19/2019] [Indexed: 05/11/2023]
Abstract
Plant mating systems play a key role in structuring genetic variation both within and between species. In hybrid zones, the outcomes and dynamics of hybridization are usually interpreted as the balance between gene flow and selection against hybrids. Yet, mating systems can introduce selective forces that alter these expectations; with diverse outcomes for the level and direction of gene flow depending on variation in outcrossing and whether the mating systems of the species pair are the same or divergent. We present a survey of hybridization in 133 species pairs from 41 plant families and examine how patterns of hybridization vary with mating system. We examine if hybrid zone mode, level of gene flow, asymmetries in gene flow and the frequency of reproductive isolating barriers vary in relation to mating system/s of the species pair. We combine these results with a simulation model and examples from the literature to address two general themes: (1) the two-way interaction between introgression and the evolution of reproductive systems, and (2) how mating system can facilitate or restrict interspecific gene flow. We conclude that examining mating system with hybridization provides unique opportunities to understand divergence and the processes underlying reproductive isolation.
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Affiliation(s)
- Melinda Pickup
- Institute of Science and Technology AustriaAm Campus 1Klosterneuburg3400Austria
| | - Yaniv Brandvain
- Department of Plant and Microbial BiologyUniversity of Minnesota1500 Gortner AveSt Paul, MinneapolisMN55108USA
| | - Christelle Fraïsse
- Institute of Science and Technology AustriaAm Campus 1Klosterneuburg3400Austria
| | - Sarah Yakimowski
- Department of BiologyQueen's University116 Barrie StKingstonONK7L 3N6Canada
| | - Nicholas H. Barton
- Institute of Science and Technology AustriaAm Campus 1Klosterneuburg3400Austria
| | - Tanmay Dixit
- Department of ZoologyUniversity of CambridgeDowning StreetCambridgeCB2 3EJUK
| | - Christian Lexer
- Department of Botany and Biodiversity ResearchFaculty of Life SciencesUniversity of ViennaA‐1030ViennaAustria
| | - Eva Cereghetti
- Institute of Science and Technology AustriaAm Campus 1Klosterneuburg3400Austria
| | - David L. Field
- Department of Botany and Biodiversity ResearchFaculty of Life SciencesUniversity of ViennaA‐1030ViennaAustria
- School of ScienceEdith Cowan University270 Joondalup DriveJoondalupWestern Australia6027Australia
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