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Penitente M, Goes CAG, dos Santos RZ, Utsunomia R, Foresti F, Porto-Foresti F. Evolutionary dynamics of the B chromosomes in the fish species Prochiloduslineatus Valenciennes, 1837 of the Paraná River Basin. COMPARATIVE CYTOGENETICS 2025; 19:1-12. [PMID: 39882391 PMCID: PMC11773350 DOI: 10.3897/compcytogen.19.135127] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 08/20/2024] [Accepted: 10/05/2024] [Indexed: 01/31/2025]
Abstract
The fish species Prochiloduslineatus has an interesting B chromosome system, with three morphological types as acrocentric, metacentric, and submetacentric. However, most cytogenetic studies on this species are restricted to the natural population of the Mogi Guaçu River. Given this, the present work aimed to study the structure karyotypic profile as well as the occurrence of supernumeraries in P.lineatus in several localities in the Paraná River basin, where this species is abundant. The results obtained showed a predominantly conserved karyotypic macrostructure and the presence of B chromosomes in all the seven localities studied, with the exception of the Apa River. Additionally, new variants of morphological characteristics were found in the population of the Batalha River (Reginópolis). These results allow us to infer that there is a large occurrence of B chromosomes in this species, with important differences in B chromosome frequency between the populations, especially in acrocentric and submetacentric B variants. Considering the possible origin and evolution of B chromosomes in P.lineatus, our results allow us to describe the dispersion of metacentric B variants, in contrast with the elimination observed in acrocentric and submetacentric variants.
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Affiliation(s)
- Manolo Penitente
- Universidade do Estado de Mato Grosso (UNEMAT), Rua Rui Barbosa, Diamantino, Mato Grosso, BrazilUniversidade do Estado de Mato Grosso (UNEMAT)DiamantinoBrazil
| | - Caio Augusto Gomes Goes
- Faculdade de Ciências, Universidade Estadual Paulista (UNESP), Avenida Edmundo Carrijo Coube, Bauru, SP, BrazilUniversidade Estadual Paulista (UNESP)BauruBrazil
| | - Rodrigo Zeni dos Santos
- Faculdade de Ciências, Universidade Estadual Paulista (UNESP), Avenida Edmundo Carrijo Coube, Bauru, SP, BrazilUniversidade Estadual Paulista (UNESP)BauruBrazil
| | - Ricardo Utsunomia
- Faculdade de Ciências, Universidade Estadual Paulista (UNESP), Avenida Edmundo Carrijo Coube, Bauru, SP, BrazilUniversidade Estadual Paulista (UNESP)BauruBrazil
| | - Fausto Foresti
- Departamento de Morfologia, Instituto de Biociências, Universidade Estadual Paulista - UNESP, Distrito de Rubião Junior, 18618-970, Botucatu, SP, BrazilUniversidade Estadual Paulista - UNESPBotucatuBrazil
| | - Fabio Porto-Foresti
- Faculdade de Ciências, Universidade Estadual Paulista (UNESP), Avenida Edmundo Carrijo Coube, Bauru, SP, BrazilUniversidade Estadual Paulista (UNESP)BauruBrazil
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2
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Vea IM, de la Filia AG, Jaron KS, Barlow SEJ, Herbette M, Mongue AJ, Nelson R, Ruiz-Ruano FJ, Ross L. The B Chromosome of Pseudococcus viburni: A Selfish Chromosome that Exploits Whole-Genome Meiotic Drive. Genome Biol Evol 2025; 17:evae257. [PMID: 39878751 PMCID: PMC11776215 DOI: 10.1093/gbe/evae257] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 11/19/2024] [Indexed: 01/31/2025] Open
Abstract
Meiosis is generally a fair process: each chromosome has a 50% chance of being included into each gamete. However, meiosis can become aberrant with some chromosomes having a higher chance of making it into gametes than others. Yet, why and how such systems evolve remains unclear. Here, we study the unusual reproductive genetics of mealybugs, where only maternal-origin chromosomes are included in gametes during male meiosis, while paternal chromosomes are eliminated. One species-Pseudococcus viburni-has a segregating B chromosome that drives by escaping paternal genome elimination. We present whole genome and gene expression data from lines with and without B chromosomes. We identify B-linked sequences including 204 protein-coding genes and a satellite repeat that makes up a significant proportion of the chromosome. The few paralogs between the B and the core genome are distributed throughout the genome, arguing against a simple, or at least recent, chromosomal duplication of one of the autosomes to create the B. We do, however, find one 373 kb region containing 146 genes that appears to be a recent translocation. Finally, we show that while many B-linked genes are expressed during meiosis, most of these are encoded on the recently translocated region. Only a small number of B-exclusive genes are expressed during meiosis. Of these, only one was overexpressed during male meiosis, which is when the drive occurs: an acetyltransferase involved in H3K56Ac, which has a putative role in meiosis and is, therefore, a promising candidate for further studies.
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Affiliation(s)
- Isabelle M Vea
- Department of Biological Sciences, University of Illinois at Chicago, Chicago, Illinois, USA
- School of Biological Sciences, Institute of Ecology and Evolution, The University of Edinburgh, Edinburgh EH9 3FL, UK
| | - Andrés G de la Filia
- School of Biological Sciences, Institute of Ecology and Evolution, The University of Edinburgh, Edinburgh EH9 3FL, UK
| | - Kamil S Jaron
- Tree of Life Programme, Wellcome Sanger Institute, Hinxton CB10 1SA, UK
| | - Scott E J Barlow
- School of Biological Sciences, Institute of Ecology and Evolution, The University of Edinburgh, Edinburgh EH9 3FL, UK
| | - Marion Herbette
- School of Biological Sciences, Institute of Ecology and Evolution, The University of Edinburgh, Edinburgh EH9 3FL, UK
| | - Andrew J Mongue
- Department of Entomology and Nematology, University of Florida, Gainesville, FL 32608, USA
| | - Ross Nelson
- School of Biological Sciences, Institute of Ecology and Evolution, The University of Edinburgh, Edinburgh EH9 3FL, UK
| | - Francisco J Ruiz-Ruano
- School of Biological Sciences, University of East Anglia, Norwich NR4 7TJ, UK
- Department of Organismal Biology, Uppsala University, Uppsala, Sweden
| | - Laura Ross
- School of Biological Sciences, Institute of Ecology and Evolution, The University of Edinburgh, Edinburgh EH9 3FL, UK
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3
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Silva DMZA, Castro JP, Goes CAG, Utsunomia R, Vidal MR, Nascimento CN, Lasmar LF, Paim FG, Soares LB, Oliveira C, Porto-Foresti F, Artoni RF, Foresti F. B Chromosomes in Psalidodon scabripinnis (Characiformes, Characidae) Species Complex. Animals (Basel) 2022; 12:2174. [PMID: 36077895 PMCID: PMC9454733 DOI: 10.3390/ani12172174] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/24/2022] [Revised: 08/19/2022] [Accepted: 08/19/2022] [Indexed: 11/24/2022] Open
Abstract
B chromosomes are extra-genomic components of cells found in individuals and in populations of some eukaryotic organisms. They have been described since the first observations of chromosomes, but several aspects of their biology remain enigmatic. Despite being present in hundreds of fungi, plants, and animal species, only a small number of B chromosomes have been investigated through high-throughput analyses, revealing the remarkable mechanisms employed by these elements to ensure their maintenance. Populations of the Psalidodon scabripinnis species complex exhibit great B chromosome diversity, making them a useful material for various analyses. In recent years, important aspects of their biology have been revealed. Here, we review these studies presenting a comprehensive view of the B chromosomes in the P. scabripinnis complex and a new hypothesis regarding the role of the B chromosome in the speciation process.
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Affiliation(s)
- Duílio M. Z. A. Silva
- Laboratory of Biology and Genetics of Fishes, Department of Structural and Functional Biology, Institute of Biosciences, São Paulo State University, Botucatu 18618-970, SP, Brazil
| | - Jonathan P. Castro
- Post-Graduate Program in Evolutionary Genetics and Molecular Biology, Department of Genetics and Evolution, Federal University of Sao Carlos, Sao Carlos 13565-905, SP, Brazil
- Laboratory of Evolutionary Genetics, Department of Structural, Molecular and Genetic Biology, State University of Ponta Grossa, Ponta Grossa 84030-900, PR, Brazil
| | - Caio A. G. Goes
- Laboratory of Fish Genetics, Department of Biological Sciences, Faculty of Sciences, São Paulo State University, Bauru 17033-360, SP, Brazil
| | - Ricardo Utsunomia
- Laboratory of Fish Genetics, Department of Biological Sciences, Faculty of Sciences, São Paulo State University, Bauru 17033-360, SP, Brazil
- Laboratory of Fish Genetics, Department of Genetics, Institute of Biological Sciences and Health, Federal Rural University of Rio de Janeiro, Seropedica 23890-000, RJ, Brazil
| | - Mateus R. Vidal
- Laboratory of Biology and Genetics of Fishes, Department of Structural and Functional Biology, Institute of Biosciences, São Paulo State University, Botucatu 18618-970, SP, Brazil
| | - Cristiano N. Nascimento
- Laboratory of Biology and Genetics of Fishes, Department of Structural and Functional Biology, Institute of Biosciences, São Paulo State University, Botucatu 18618-970, SP, Brazil
| | - Lucas F. Lasmar
- Laboratory of Biology and Genetics of Fishes, Department of Structural and Functional Biology, Institute of Biosciences, São Paulo State University, Botucatu 18618-970, SP, Brazil
| | - Fabilene G. Paim
- Laboratory of Biology and Genetics of Fishes, Department of Structural and Functional Biology, Institute of Biosciences, São Paulo State University, Botucatu 18618-970, SP, Brazil
| | - Letícia B. Soares
- Laboratory of Biology and Genetics of Fishes, Department of Structural and Functional Biology, Institute of Biosciences, São Paulo State University, Botucatu 18618-970, SP, Brazil
| | - Claudio Oliveira
- Laboratory of Biology and Genetics of Fishes, Department of Structural and Functional Biology, Institute of Biosciences, São Paulo State University, Botucatu 18618-970, SP, Brazil
| | - Fábio Porto-Foresti
- Laboratory of Fish Genetics, Department of Biological Sciences, Faculty of Sciences, São Paulo State University, Bauru 17033-360, SP, Brazil
| | - Roberto F. Artoni
- Post-Graduate Program in Evolutionary Genetics and Molecular Biology, Department of Genetics and Evolution, Federal University of Sao Carlos, Sao Carlos 13565-905, SP, Brazil
- Laboratory of Evolutionary Genetics, Department of Structural, Molecular and Genetic Biology, State University of Ponta Grossa, Ponta Grossa 84030-900, PR, Brazil
| | - Fausto Foresti
- Laboratory of Biology and Genetics of Fishes, Department of Structural and Functional Biology, Institute of Biosciences, São Paulo State University, Botucatu 18618-970, SP, Brazil
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Goes CAG, Silva DMZDA, Utsunomia R, Nascimento NFD, Yasui GS, Senhorini JA, Hashimoto DT, Artoni RF, Foresti F, Porto-Foresti F. Sex-Dependent Inheritance of B Chromosomes in Psalidodon paranae (Teleostei, Characiformes) Revealed by Directed Crossings. Zebrafish 2021; 18:363-368. [PMID: 34935496 DOI: 10.1089/zeb.2021.0053] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
B chromosomes are additional dispensable elements to the standard chromosomal set of an organism. In most cases, their transmission differs from Mendelian patterns, leading to their accumulation or extinction. The present study aimed to describe, for the first time, the transmission pattern of B chromosome in a population of Psalidodon paranae through directed crosses, as well as to analyze the populational dynamics of B chromosome. Our results revealed the possible elimination of B chromosome in crossings where only females were B-carriers, with a mean transmission rate (kB) of 0.149; however, kB was significantly higher in crossings involving male B-carriers (kB = 0.328-0.450). Moreover, we observed an increase in the frequency of B chromosomes in the natural population of P. paranae in the last two decades. These apparently contradictory results can make sense if the B chromosome provides adaptive advantages to their carriers. Here, we observed a differential transmission of B chromosomes in each sex of parental individuals, with higher transmission rates in crossing involving males B-carriers, in addition to describe the temporal changes of B chromosome frequency in P. paranae.
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Affiliation(s)
- Caio Augusto Gomes Goes
- Departamento de Ciências Biológicas, Faculdade de Ciências, Universidade Estadual Paulista (UNESP) "Júlio de Mesquita Filho," Bauru, Brazil
| | | | - Ricardo Utsunomia
- Departamento de Genética, Instituto de Ciências Biológicas e da Saúde, ICBS, Universidade Federal Rural do Rio de Janeiro, Seropédica, Brazil
| | | | - George Shigueki Yasui
- Centro nacional de Pesquisa e Conservação da Biota Aquática Continental (CEPTA-ICMBIO), Pirassununga, Brazil
| | - José Augusto Senhorini
- Centro nacional de Pesquisa e Conservação da Biota Aquática Continental (CEPTA-ICMBIO), Pirassununga, Brazil
| | - Diogo Teruo Hashimoto
- Centro de Aquicultura da UNESP, Universidade Estadual Paulista "Júlio de Mesquita Filho" (UNESP), Jaboticabal, Brazil
| | - Roberto Ferreira Artoni
- Departamento de Biologia Estrutural, Molecular e Genética, Setor de Ciências Biológicas e da Saúde, Universidade Estadual de Ponta Grossa, Ponta Grossa, Brazil
| | - Fausto Foresti
- Departamento de Biologia Estrutural e Funcional, Instituto de Biociências, Universidade Estadual Paulista "Júlio de Mesquita Filho" (UNESP), Botucatu, Brazil
| | - Fábio Porto-Foresti
- Departamento de Ciências Biológicas, Faculdade de Ciências, Universidade Estadual Paulista (UNESP) "Júlio de Mesquita Filho," Bauru, Brazil
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5
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Verspoor RL, Price TAR, Wedell N. Selfish genetic elements and male fertility. Philos Trans R Soc Lond B Biol Sci 2020; 375:20200067. [PMID: 33070738 PMCID: PMC7661447 DOI: 10.1098/rstb.2020.0067] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 06/19/2020] [Indexed: 12/14/2022] Open
Abstract
Selfish genetic elements (SGEs) are diverse and near ubiquitous in Eukaryotes and can be potent drivers of evolution. Here, we discuss SGEs that specifically act on sperm to gain a transmission advantage to the next generation. The diverse SGEs that affect sperm often impose costs on carrier males, including damaging ejaculates, skewing offspring sex ratios and in particular reducing sperm-competitive success of SGE-carrying males. How males and females tolerate and mitigate against these costs is a dynamic and expanding area of research. The intense intra-genomic conflict that these selfish elements generate could also have implications for male fertility and spermatogenesis more widely. This article is part of the theme issue 'Fifty years of sperm competition'.
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Affiliation(s)
- Rudi L. Verspoor
- Institute of Infection, Veterinary and Ecological Sciences, University of Liverpool, Liverpool L69 7ZB, UK
| | - Tom A. R. Price
- Institute of Infection, Veterinary and Ecological Sciences, University of Liverpool, Liverpool L69 7ZB, UK
| | - Nina Wedell
- Biosciences, University of Exeter, Penryn Campus, Penryn TR10 9FE, UK
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6
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Melo AS, Cruz GAS, Félix AP, Rocha MF, Loreto V, Moura RC. Wide dispersion of B chromosomes in Rhammatocerus brasiliensis (Orthoptera, Acrididae). Genet Mol Biol 2020; 43:e20190077. [PMID: 32542305 PMCID: PMC7295183 DOI: 10.1590/1678-4685-gmb-2019-0077] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/13/2019] [Accepted: 07/16/2019] [Indexed: 01/01/2023] Open
Abstract
The grasshopper Rhammatocerus brasiliensis shows polymorphism of B chromosomes, but the magnitude of B-chromosome occurrence and the factors that may contribute to their dispersion in the species remain unknown thus far. The present study analyzed the occurrence and dispersion of B chromosomes in R. brasiliensis individuals from 21 populations widely distributed in the Brazilian Northeast. The genetic connectivity between 10 populations was verified through analysis of ISSR markers from 200 individuals. Of the 21 populations, 19 presented individuals with one B chromosome, three with two, and one with three B chromosomes. The B chromosome is of medium size and constitutive heterochromatin (CH) located in the pericentromeric region. A variant B chromosome was observed in three populations, similar in size to that of chromosome X, gap and CH, and located in the terminal region. B chromosome frequencies in different populations varied from 0% to 18,8%, mean 8,5%. The wide distribution of the B chromosome is likely a consequence of the positive gene flow among the analyzed populations. B-chromosome occurrence in populations of R. brasiliensis possibly follows the population genetic structure of the species and, owing to the existence of a variant, its origin may not be recent.
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Affiliation(s)
- Adriana S Melo
- Universidade de Pernambuco (UPE), Instituto de Ciências Biológicas, Laboratório de Biodiversidade e Genética de Insetos, Recife, PE, Brazil
| | - Geyner A S Cruz
- Universidade de Pernambuco (UPE), Instituto de Ciências Biológicas, Laboratório de Biodiversidade e Genética de Insetos, Recife, PE, Brazil
- Universidade de Pernambuco (UPE), Laboratório de Biodiversidade e Genética Evolutiva, Campus Petrolina, Petrolina, PE, Brazil
| | - Aline P Félix
- Universidade de Pernambuco (UPE), Instituto de Ciências Biológicas, Laboratório de Biodiversidade e Genética de Insetos, Recife, PE, Brazil
| | - Marília F Rocha
- Universidade de Pernambuco (UPE), Instituto de Ciências Biológicas, Laboratório de Biodiversidade e Genética de Insetos, Recife, PE, Brazil
| | - Vilma Loreto
- Universidade Federal de Pernambuco (UFPE), Departamento de Genética, Laboratório de Genética Animal e Humana e Citogenética, Recife, PE, Brazil
| | - Rita C Moura
- Universidade de Pernambuco (UPE), Instituto de Ciências Biológicas, Laboratório de Biodiversidade e Genética de Insetos, Recife, PE, Brazil
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7
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Manrique-Poyato MI, Cabrero J, López-León MD, Perfectti F, Gómez R, Camacho JPM. Interpopulation spread of a parasitic B chromosome is unlikely through males in the grasshopper Eyprepocnemis plorans. Heredity (Edinb) 2020; 124:197-206. [PMID: 31285567 PMCID: PMC6906446 DOI: 10.1038/s41437-019-0248-5] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/10/2019] [Revised: 06/11/2019] [Accepted: 06/25/2019] [Indexed: 11/09/2022] Open
Abstract
The near-neutral model of B chromosome evolution predicts that population invasion is quite fast. To test this prediction, in 1994, we introduced males of the grasshopper Eyprepocnemis plorans from a B-carrying population into a B-lacking population and monitored the evolution of B-chromosome frequency up to 2013. We observed fluctuating very low B frequency across years but, remarkably, the B chromosome introduced (the B2 variant) was found up to 1996 only, whereas the B1 variant was present from 1996 onwards, presumably introduced by fishermen using E. plorans males as bait. Effective introgression of genetic material from the donor population was evidenced by the presence of a satellite DNA on autosome 9 (up to 1999) and the presence of one individual in 2006 showing an ISSR marker profile being highly similar to that found in the donor population. This indicated that the males introduced by us effectively mated with resident females, but donor genes rapidly decreased in frequency after this non-recurrent migration event. Taken together, our results indicated: (i) that the non-recurrent migration event had a slight, transient genetic effect on the recipient population, which was diluted in only a few generations; and (ii) that even with recurrent migration (forced by fishermen) the B chromosome failed to increase in frequency. Bearing in mind that B chromosomes in this species drive through females only, we hypothesize that B chromosomes most likely failed invasion in both migration events because the migrating sex shows no B-drive.
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Affiliation(s)
| | - Josefa Cabrero
- Departamento de Genética, Facultad de Ciencias, Universidad de Granada, Avda. Fuentenueva s/n, 18071, Granada, Spain
| | - María Dolores López-León
- Departamento de Genética, Facultad de Ciencias, Universidad de Granada, Avda. Fuentenueva s/n, 18071, Granada, Spain
| | - Francisco Perfectti
- Departamento de Genética, Facultad de Ciencias, Universidad de Granada, Avda. Fuentenueva s/n, 18071, Granada, Spain
| | - Ricardo Gómez
- Departamento de Ciencia y Tecnología Agroforestal, E.T.S. de Ingenieros Agrónomos, Universidad de Castilla La Mancha, 02071, Albacete, Spain
| | - Juan Pedro M Camacho
- Departamento de Genética, Facultad de Ciencias, Universidad de Granada, Avda. Fuentenueva s/n, 18071, Granada, Spain.
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8
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Gene expression changes elicited by a parasitic B chromosome in the grasshopper Eyprepocnemis plorans are consistent with its phenotypic effects. Chromosoma 2019; 128:53-67. [PMID: 30617552 DOI: 10.1007/s00412-018-00689-y] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/24/2018] [Revised: 12/20/2018] [Accepted: 12/21/2018] [Indexed: 01/16/2023]
Abstract
Parasitism evokes adaptive physiological changes in the host, many of which take place through gene expression changes. This response can be more or less local, depending on the organ or tissue affected by the parasite, or else systemic when the parasite affects the entire host body. The most extreme of the latter cases is intragenomic parasitism, where the parasite is present in all host nuclei as any other genomic element. Here, we show the molecular crosstalk between a parasitic chromosome (also named B chromosome) and the host genome, manifested through gene expression changes. The transcriptome analysis of 0B and 1B females of the grasshopper Eyprepocnemis plorans, validated by a microarray experiment performed on four B-lacking and five B-carrying females, revealed changes in gene expression for 188 unigenes being consistent in both experiments. Once discarded B-derived transcripts, there were 46 differentially expressed genes (30 up- and 16 downregulated) related with the adaptation of the host genome to the presence of the parasitic chromosome. Interestingly, the functions of these genes could explain some of the most important effects of B chromosomes, such as nucleotypic effects derived from the additional DNA they represent, chemical defense and detoxification, protein modification and response to stress, ovary function, and regulation of gene expression. Collectively, these changes uncover an intimate host-parasite interaction between A and B chromosomes during crucial steps of gene expression and protein function.
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9
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Transmission and Drive Involving Parasitic B Chromosomes. Genes (Basel) 2018; 9:genes9080388. [PMID: 30065230 PMCID: PMC6115934 DOI: 10.3390/genes9080388] [Citation(s) in RCA: 26] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/25/2018] [Revised: 07/26/2018] [Accepted: 07/26/2018] [Indexed: 11/28/2022] Open
Abstract
B chromosomes (Bs) are enigmatic additional elements in the genomes of thousands of species of plants, animals, and fungi. How do these non-essential, harmful, and parasitic chromosomes maintain their presence in their hosts, making demands on all the essential functions of their host genomes? The answer seems to be that they have mechanisms of drive which enable them to enhance their transmission rates by various processes of non-mendelian inheritance. It is also becoming increasingly clear that the host genomes are developing their own mechanisms to resist the impact of the harmful effects of the Bs.
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