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Toivonen J, Fromhage L. Hybridization selects for prime-numbered life cycles in Magicicada: An individual-based simulation model of a structured periodical cicada population. Ecol Evol 2020; 10:5259-5269. [PMID: 32607149 PMCID: PMC7319174 DOI: 10.1002/ece3.6270] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/14/2020] [Revised: 03/18/2020] [Accepted: 03/23/2020] [Indexed: 11/21/2022] Open
Abstract
We investigate competition between separate periodical cicada populations each possessing different life-cycle lengths. We build an individual-based model to simulate the cicada life cycle and allow random migrations to occur between patches inhabited by the different populations. We show that if hybridization between different cycle lengths produces offspring that have an intermediate life-cycle length, then predation acts disproportionately to select against the hybrid offspring. This happens because they emerge in low densities without the safety-in-numbers provided by either parent population. Thus, prime-numbered life cycles that can better avoid hybridization are favored. However, we find that this advantage of prime-numbered cycles occurs only if there is some mechanism that can occasionally synchronize emergence between local populations in sufficiently many patches.
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Affiliation(s)
- Jaakko Toivonen
- Department of Computer ScienceUniversity of HelsinkiHelsinkiFinland
- Department of Biological and Environmental ScienceUniversity of JyväskyläJyväskyläFinland
| | - Lutz Fromhage
- Department of Biological and Environmental ScienceUniversity of JyväskyläJyväskyläFinland
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Du Z, Hasegawa H, Cooley JR, Simon C, Yoshimura J, Cai W, Sota T, Li H. Mitochondrial Genomics Reveals Shared Phylogeographic Patterns and Demographic History among Three Periodical Cicada Species Groups. Mol Biol Evol 2019; 36:1187-1200. [PMID: 30850829 PMCID: PMC6526903 DOI: 10.1093/molbev/msz051] [Citation(s) in RCA: 62] [Impact Index Per Article: 12.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022] Open
Abstract
The mass application of whole mitogenome (MG) sequencing has great potential for resolving complex phylogeographic patterns that cannot be resolved by partial mitogenomic sequences or nuclear markers. North American periodical cicadas (Magicicada) are well known for their periodical mass emergence at 17- and 13-year intervals in the north and south, respectively. Magicicada comprises three species groups, each containing one 17-year species and one or two 13-year species. Within each life cycle, single-aged cohorts, called broods, of periodical cicadas emerge in different years, and most broods contain members of all three species groups. There are 12 and three extant broods of 17- and 13-year cicadas, respectively. The phylogeographic relationships among the populations and broods within the species groups have not been clearly resolved. We analyzed 125 whole MG sequences from all broods and seven species within three species groups to ascertain the divergence history of the geographic and allochronic populations and their life cycles. Our mitogenomic phylogeny analysis clearly revealed that each of the three species groups had largely similar phylogeographic subdivisions (east, middle, and west) and demographic histories (rapid population expansion after the last glacial period). The mitogenomic phylogeny also partly resolved the brood diversification process, which could be explained by hypothetical temporary life cycle shifts, and showed that none of the 13- and 17-year species within the species groups was monophyletic, possibly due to gene flow between them. Our findings clearly reveal phylogeographic structures in the three Magicicada species groups, demonstrating the advantage of whole MG sequence data in phylogeographic studies.
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Affiliation(s)
- Zhenyong Du
- Department of Entomology and MOA Key Lab of Pest Monitoring and Green Management, College of Plant Protection, China Agricultural University, Beijing, China
| | - Hiroki Hasegawa
- Department of Zoology, Graduate School of Science, Kyoto University, Sakyo, Kyoto, Japan
| | - John R Cooley
- Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, CT
| | - Chris Simon
- Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, CT
| | - Jin Yoshimura
- Graduate School of Science and Technology and Department of Mathematical and Systems Engineering, Shizuoka University, Hamamatsu, Japan.,Department of Environmental and Forest Biology, State University of New York College of Environmental Science and Forestry, Syracuse, NY.,Marine Biosystems Research Center, Chiba University, Kamogawa, Chiba, Japan
| | - Wanzhi Cai
- Department of Entomology and MOA Key Lab of Pest Monitoring and Green Management, College of Plant Protection, China Agricultural University, Beijing, China
| | - Teiji Sota
- Department of Zoology, Graduate School of Science, Kyoto University, Sakyo, Kyoto, Japan
| | - Hu Li
- Department of Entomology and MOA Key Lab of Pest Monitoring and Green Management, College of Plant Protection, China Agricultural University, Beijing, China
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Loxdale HD. Aspects, Including Pitfalls, of Temporal Sampling of Flying Insects, with Special Reference to Aphids. Insects 2018; 9:E153. [PMID: 30388726 PMCID: PMC6316496 DOI: 10.3390/insects9040153] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Subscribe] [Scholar Register] [Received: 09/10/2018] [Revised: 10/26/2018] [Accepted: 10/26/2018] [Indexed: 12/31/2022]
Abstract
Since the advent and widespread use of high-resolution molecular markers in the late 1970s, it is now well established that natural populations of insects are not necessarily homogeneous genetically and show variations at different spatial scales due to a variety of reasons, including hybridization/introgression events. In a similar vein, populations of insects are not necessarily homogenous in time, either over the course of seasons or even within a single season. This of course has profound consequences for surveys examining, for whatever reason/s, the temporal population patterns of insects, especially flying insects as mostly discussed here. In the present article, the topics covered include climate and climate change; changes in ecological niches due to changes in available hosts, i.e., essentially, adaptation events; hybridization influencing behaviour⁻host shifts; infection by pathogens and parasites/parasitoids; habituation to light, sound and pheromone lures; chromosomal/genetic changes affecting physiology and behaviour; and insecticide resistance. If such phenomena-i.e., aspects and pitfalls-are not considered during spatio-temporal study programmes, which is even more true in the light of the recent discovery of morphologically similar/identical cryptic species, then the conclusions drawn in terms of the efforts to combat pest insects or conserve rare and endangered species may be in error and hence end in failure.
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Affiliation(s)
- Hugh D Loxdale
- School of Biosciences, Cardiff University, The Sir Martin Evans Building, Museum Avenue, Cardiff CF10 3AX, Wales, UK.
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Cooley JR, Arguedas N, Bonaros E, Bunker G, Chiswell SM, DeGiovine A, Edwards M, Hassanieh D, Haji D, Knox J, Kritsky G, Mills C, Mozgai D, Troutman R, Zyla J, Hasegawa H, Sota T, Yoshimura J, Simon C. The periodical cicada four-year acceleration hypothesis revisited and the polyphyletic nature of Brood V, including an updated crowd-source enhanced map (Hemiptera: Cicadidae: Magicicada). PeerJ 2018; 6:e5282. [PMID: 30083444 PMCID: PMC6074776 DOI: 10.7717/peerj.5282] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/04/2018] [Accepted: 06/26/2018] [Indexed: 11/20/2022] Open
Abstract
The periodical cicadas of North America (Magicicada spp.) are well-known for their long life cycles of 13 and 17 years and their mass synchronized emergences. Although periodical cicada life cycles are relatively strict, the biogeographic patterns of periodical cicada broods, or year-classes, indicate that they must undergo some degree of life cycle switching. We present a new map of periodical cicada Brood V, which emerged in 2016, and demonstrate that it consists of at least four distinct parts that span an area in the United States stretching from Ohio to Long Island. We discuss mtDNA haplotype variation in this brood in relation to other periodical cicada broods, noting that different parts of this brood appear to have different origins. We use this information to refine a hypothesis for the formation of periodical cicada broods by 1- and 4-year life cycle jumps.
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Affiliation(s)
- John R Cooley
- College of Integrative Sciences, Wesleyan University, Middletown, CT, United States of America.,Department of Ecology and Evolutionary Biology, The University of Connecticut, Storrs, CT, United States of America
| | - Nidia Arguedas
- Cleveland Metroparks, Cleveland, OH, United States of America
| | | | - Gerry Bunker
- Massachusetts Cicadas, Marlborough, MA, United States of America
| | - Stephen M Chiswell
- National Institute of Water and Atmospheric Research, Wellington, New Zealand
| | | | - Marten Edwards
- Department of Biology, Muhlenberg College, Allentown, PA, United States of America
| | - Diane Hassanieh
- Department of Ecology and Evolutionary Biology, The University of Connecticut, Storrs, CT, United States of America
| | - Diler Haji
- Department of Ecology and Evolutionary Biology, The University of Connecticut, Storrs, CT, United States of America
| | - John Knox
- Department of Biology, Washington and Lee University, Lexington, VA, United States of America
| | - Gene Kritsky
- Department of Biology, Mount St. Joseph University, Cincinnati, OH, United States of America
| | - Carolyn Mills
- Research Services, The University of Connecticut Libraries, Storrs, CT, United States of America
| | - Dan Mozgai
- Cicada mania, NJ, United States of America
| | | | - John Zyla
- Mid-Atlantic Cicadas, MD, United States of America
| | - Hiroki Hasegawa
- Department of Zoology, Graduate School of Science, Kyoto University, Kyoto, Japan
| | - Teiji Sota
- Department of Zoology, Graduate School of Science, Kyoto University, Kyoto, Japan
| | - Jin Yoshimura
- Graduate School of Science and Technology, Shizuoka University, Hamamatsu, Japan
| | - Chris Simon
- Department of Ecology and Evolutionary Biology, The University of Connecticut, Storrs, CT, United States of America
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Fujisawa T, Koyama T, Kakishima S, Cooley JR, Simon C, Yoshimura J, Sota T. Triplicate parallel life cycle divergence despite gene flow in periodical cicadas. Commun Biol 2018; 1:26. [PMID: 30271912 PMCID: PMC6123741 DOI: 10.1038/s42003-018-0025-7] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/20/2017] [Accepted: 03/01/2018] [Indexed: 01/24/2023] Open
Abstract
Periodical cicadas comprise three species groups containing three pairs of 13- and 17-year life cycle species showing parallel divergence, along with a more anciently diverged 13-year species (Magicicda tredecim). The mechanism and genetic basis of this parallel divergence is unknown. Here we use orthologous transcriptome sequences to explore the demographic processes and genomic evolution associated with parallel life cycle divergence. The three 13- and 17-year species pairs have similar demographic histories, and the two life cycles diverged 200,000-100,000 years ago. Interestingly, these life cycle differences have been maintained despite substantial gene flow between 13- and 17-year species within species groups, which is possible during co-emergences. Sequence divergence between 13- and 17-year species in each species group (excluding M. tredecim) is minimal, and we find no shared divergent single-nucleotide polymorphisms (SNPs) or loci associated with all instances of life cycle divergence. The two life cycles may be controlled by highly limited genomic differences.
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Affiliation(s)
- Tomochika Fujisawa
- Department of Zoology, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502 Japan
| | - Takuya Koyama
- Department of Zoology, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502 Japan
| | - Satoshi Kakishima
- Graduate School of Science and Technology, Shizuoka University, Hamamatsu, 432-8561 Japan
- Department of Botany, National Museum of Nature and Science, Tsukuba, 305-0005 Japan
| | - John R. Cooley
- College of Integrative Sciences, Wesleyan University, Middletown, CT 06459 USA
- Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, CT 06268-3043 USA
| | - Chris Simon
- Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, CT 06268-3043 USA
| | - Jin Yoshimura
- Graduate School of Science and Technology, Shizuoka University, Hamamatsu, 432-8561 Japan
- Department of Environmental and Forest Biology, State University of New York College of Environmental Science and Forestry, Syracuse, NY 13210 USA
- Marine Biosystems Research Center, Chiba University, Uchiura, Kamogawa, Chiba 299-5502 Japan
| | - Teiji Sota
- Department of Zoology, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502 Japan
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Sota T, Yamamoto S, Cooley JR, Hill KB, Simon C, Yoshimura J. Independent divergence of 13- and 17-y life cycles among three periodical cicada lineages. Proc Natl Acad Sci U S A 2013; 110:6919-24. [PMID: 23509294 DOI: 10.1073/pnas.1220060110] [Citation(s) in RCA: 35] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The evolution of 13- and 17-y periodical cicadas (Magicicada) is enigmatic because at any given location, up to three distinct species groups (Decim, Cassini, Decula) with synchronized life cycles are involved. Each species group is divided into one 13- and one 17-y species with the exception of the Decim group, which contains two 13-y species-13-y species are Magicicada tredecim, Magicicada neotredecim, Magicicada tredecassini, and Magicicada tredecula; and 17-y species are Magicicada septendecim, Magicicada cassini, and Magicicada septendecula. Here we show that the divergence leading to the present 13- and 17-y populations differs considerably among the species groups despite the fact that each group exhibits strikingly similar phylogeographic patterning. The earliest divergence of extant lineages occurred ∼4 Mya with one branch forming the Decim species group and the other subsequently splitting 2.5 Mya to form the Cassini and Decula species groups. The earliest split of extant lineages into 13- and 17-y life cycles occurred in the Decim lineage 0.5 Mya. All three species groups experienced at least one episode of life cycle divergence since the last glacial maximum. We hypothesize that despite independent origins, the three species groups achieved their current overlapping distributions because life-cycle synchronization of invading congeners to a dominant resident population enabled escape from predation and population persistence. The repeated life-cycle divergences supported by our data suggest the presence of a common genetic basis for the two life cycles in the three species groups.
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Fergus DJ, Decarvalho TN, Shaw KL. Genetically regulated temporal variation of novel courtship elements in the Hawaiian cricket genus Laupala. Behav Genet 2011; 41:607-14. [PMID: 20878226 PMCID: PMC3086961 DOI: 10.1007/s10519-010-9397-2] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/24/2010] [Accepted: 09/13/2010] [Indexed: 10/19/2022]
Abstract
The Hawaiian cricket genus Laupala (Gryllidae: Trigonidiinae) has undergone rapid and extensive speciation, with divergence in male song and female acoustic preference playing a role in maintaining species boundaries. Recent study of interspecific differences in the diel rhythmicity of singing and mating, suggests that temporal variation in behavior may reduce gene flow between species. In addition, Laupala perform an elaborate and protracted courtship, providing potential for further temporal variation. However, whether these behavioral differences have a genetic basis or result from environmental variation is unknown. We observed courtship and mating in a common garden study of the sympatric species, Laupala cerasina and Laupala paranigra. We document interspecific differences in the onset and duration of courtship, spermatophore production rate, and diel mating rhythmicity. Our study demonstrates a genetic contribution to interspecific behavioral differences, and suggests an evolutionary pathway to the origins of novel timing phenotypes.
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Affiliation(s)
- Daniel J Fergus
- Department of Neurobiology and Behavior, Cornell University, Seeley G Mudd Hall, Ithaca, NY 14853, USA.
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Nariai Y, Hayashi S, Morita S, Umemura Y, Tainaka KI, Sota T, Cooley JR, Yoshimura J. Life cycle replacement by gene introduction under an allee effect in periodical cicadas. PLoS One 2011; 6:e18347. [PMID: 21494682 PMCID: PMC3071825 DOI: 10.1371/journal.pone.0018347] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/20/2011] [Accepted: 02/25/2011] [Indexed: 11/25/2022] Open
Abstract
Periodical cicadas (Magicicada spp.) in the USA are divided into three species groups (-decim, -cassini, -decula) of similar but distinct morphology and behavior. Each group contains at least one species with a 17-year life cycle and one with a 13-year cycle; each species is most closely related to one with the other cycle. One explanation for the apparent polyphyly of 13- and 17-year life cycles is that populations switch between the two cycles. Using a numerical model, we test the general feasibility of life cycle switching by the introduction of alleles for one cycle into populations of the other cycle. Our results suggest that fitness reductions at low population densities of mating individuals (the Allee effect) could play a role in life cycle switching. In our model, if the 13-year cycle is genetically dominant, a 17-year cycle population will switch to a 13-year cycle given the introduction of a few 13-year cycle alleles under a moderate Allee effect. We also show that under a weak Allee effect, different year-classes (“broods”) with 17-year life cycles can be generated. Remarkably, the outcomes of our models depend only on the dominance relationships of the cycle alleles, irrespective of any fitness advantages.
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Affiliation(s)
- Yukiko Nariai
- Department of Systems Engineering, Shizuoka University, Hamamatsu, Japan
| | - Saki Hayashi
- Department of Systems Engineering, Shizuoka University, Hamamatsu, Japan
- Faculty of Information Science, Shizuoka University, Hamamatsu, Japan
| | - Satoru Morita
- Department of Systems Engineering, Shizuoka University, Hamamatsu, Japan
| | - Yoshitaka Umemura
- Department of Systems Engineering, Shizuoka University, Hamamatsu, Japan
| | - Kei-ichi Tainaka
- Department of Systems Engineering, Shizuoka University, Hamamatsu, Japan
| | - Teiji Sota
- Department of Zoology, Kyoto University, Kyoto, Japan
| | - John R. Cooley
- Department of Systems Engineering, Shizuoka University, Hamamatsu, Japan
- Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, Connecticut, United States of America
| | - Jin Yoshimura
- Department of Systems Engineering, Shizuoka University, Hamamatsu, Japan
- Marine Biosystems Research Center, Chiba University, Kamogawa, Chiba, Japan
- Department of Environmental and Forest Biology, State University of New York College of Environmental Science and Forestry, Syracuse, New York, United States of America
- * E-mail:
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Tanaka Y, Yoshimura J, Simon C, Cooley JR, Tainaka K. Allee effect in the selection for prime-numbered cycles in periodical cicadas. Proc Natl Acad Sci U S A 2009; 106:8975-9. [PMID: 19451640 DOI: 10.1073/pnas.0900215106] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Periodical cicadas are well known for their prime-numbered life cycles (17 and 13 years) and their mass periodical emergences. The origination and persistence of prime-numbered cycles are explained by the hybridization hypothesis on the basis of their lower likelihood of hybridization with other cycles. Recently, we showed by using an integer-based numerical model that prime-numbered cycles are indeed selected for among 10- to 20-year cycles. Here, we develop a real-number-based model to investigate the factors affecting the selection of prime-numbered cycles. We include an Allee effect in our model, such that a critical population size is set as an extinction threshold. We compare the real-number models with and without the Allee effect. The results show that in the presence of an Allee effect, prime-numbered life cycles are most likely to persist and to be selected under a wide range of extinction thresholds.
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Abstract
Disruptive selection of life-cycle timing may cause temporal isolation directly and, ultimately, allochronic speciation. Despite the fact that segregation of the reproductive period among related species has been broadly observed across taxa, it remains controversial whether temporal isolation can function as the primary process of speciation. In the Japanese winter geometrid moth Inurois punctigera, allochronic divergence has resulted from climatic disruption of the reproductive period. In habitats with severe midwinter, two sympatric groups of moth reproduce allochronically in early and late winter. These groups are genetically diverging sister lineages and now co-occur allochronically throughout Japan. By contrast, in habitats with milder midwinter these lineages form a continuous adult period and gene flow has been facilitated between the lineages. These results, together with the fact that there is no difference in larval host use, indicate that temporal isolation has been the sole mechanism for allochronic isolation in colder habitats and that allochrony is not a by-product of other adaptations. Thus, the allochronic divergence of sympatric I. punctigera populations represents an incipient speciation process driven by midwinter disruption of the reproductive period.
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Affiliation(s)
- Satoshi Yamamoto
- Department of Zoology, Graduate School of Science, Kyoto University, Kitashirakawa-oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan.
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Abstract
Mitochondrial inheritance is generally assumed to be maternal. However, there is increasing evidence of exceptions to this rule, especially in hybrid crosses. In these cases, mitochondria are also inherited paternally, so “paternal leakage” of mitochondria occurs. It is important to understand these exceptions better, since they potentially complicate or invalidate studies that make use of mitochondrial markers. We surveyed F1 offspring of experimental hybrid crosses of the 17-year periodical cicadas Magicicada septendecim, M. septendecula, and M. cassini for the presence of paternal mitochondrial markers at various times during development (1-day eggs; 3-, 6-, 9-week eggs; 16-month old 1st and 2nd instar nymphs). We found evidence of paternal leakage in both reciprocal hybrid crosses in all of these samples. The relative difficulty of detecting paternal mtDNA in the youngest eggs and ease of detecting leakage in older eggs and in nymphs suggests that paternal mitochondria proliferate as the eggs develop. Our data support recent theoretical predictions that paternal leakage may be more common than previously estimated.
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Affiliation(s)
- Kathryn M. Fontaine
- Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, Connecticut, United States of America
| | - John R. Cooley
- Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, Connecticut, United States of America
- * To whom correspondence should be addressed. E-mail:
| | - Chris Simon
- Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, Connecticut, United States of America
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Mallet J, Beltrán M, Neukirchen W, Linares M. Natural hybridization in heliconiine butterflies: the species boundary as a continuum. BMC Evol Biol 2007; 7:28. [PMID: 17319954 PMCID: PMC1821009 DOI: 10.1186/1471-2148-7-28] [Citation(s) in RCA: 178] [Impact Index Per Article: 10.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/23/2006] [Accepted: 02/23/2007] [Indexed: 11/24/2022] Open
Abstract
BACKGROUND To understand speciation and the maintenance of taxa as separate entities, we need information about natural hybridization and gene flow among species. RESULTS Interspecific hybrids occur regularly in Heliconius and Eueides (Lepidoptera: Nymphalidae) in the wild: 26-29% of the species of Heliconiina are involved, depending on species concept employed. Hybridization is, however, rare on a per-individual basis. For one well-studied case of species hybridizing in parapatric contact (Heliconius erato and H. himera), phenotypically detectable hybrids form around 10% of the population, but for species in sympatry hybrids usually form less than 0.05% of individuals. There is a roughly exponential decline with genetic distance in the numbers of natural hybrids in collections, both between and within species, suggesting a simple "exponential failure law" of compatibility as found in some prokaryotes. CONCLUSION Hybridization between species of Heliconius appears to be a natural phenomenon; there is no evidence that it has been enhanced by recent human habitat disturbance. In some well-studied cases, backcrossing occurs in the field and fertile backcrosses have been verified in insectaries, which indicates that introgression is likely, and recent molecular work shows that alleles at some but not all loci are exchanged between pairs of sympatric, hybridizing species. Molecular clock dating suggests that gene exchange may continue for more than 3 million years after speciation. In addition, one species, H. heurippa, appears to have formed as a result of hybrid speciation. Introgression may often contribute to adaptive evolution as well as sometimes to speciation itself, via hybrid speciation. Geographic races and species that coexist in sympatry therefore form part of a continuum in terms of hybridization rates or probability of gene flow. This finding concurs with the view that processes leading to speciation are continuous, rather than sudden, and that they are the same as those operating within species, rather than requiring special punctuated effects or complete allopatry. Although not qualitatively distinct from geographic races, nor "real" in terms of phylogenetic species concepts or the biological species concept, hybridizing species of Heliconius are stably distinct in sympatry, and remain useful groups for predicting morphological, ecological, behavioural and genetic characteristics.
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Affiliation(s)
- James Mallet
- Galton Laboratory, University College London, Wolfson House, 4 Stephenson Way, London NW1 2HE, UK
- Department of Entomology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK
- Smithsonian Tropical Research Institute, Balboa, Apartado 2072, Panamá
| | - Margarita Beltrán
- Galton Laboratory, University College London, Wolfson House, 4 Stephenson Way, London NW1 2HE, UK
- Smithsonian Tropical Research Institute, Balboa, Apartado 2072, Panamá
| | | | - Mauricio Linares
- Departamento de Ciencias Biológicas, Instituto de Genética, Universidad de los Andes, Carrera 1E No 18A10, Bogotá, Colombia
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