On 2014 Jan 29, Peter Beerli commented:

Inheritance patterns in diploid and triploid water frog hybrids (Pelophylax esculentus) – a comment on Pruvost et al.

Jörg Plötner^1, Gaston-Denis Guex^2, Peter Beerli^3, and Thomas Uzzell⁴

(Addresses at the end)

Pruvost et al. (2013) described the gamete production and ploidy of water frogs from five populations in Germany, Poland, and Slovakia. Two populations were composed of P. lessonae (genotype LL), P. ridibundus (RR) and their hybridogenetic hybrid P. esculentus (LR, LLR, RRL) whereas three populations consisted of only diploid LR individuals and triploids (LLR, RRL). Based on crossing experiments involving 64 P. esculentus and the analysis of microsatellites, the authors found that diploid males (genotype LR) produced haploid gametes with a ridibundus (R) genome whereas LR females usually produced diploid eggs containing both parental genomes (LR gametes). Moreover, most of the triploid individuals transmitted to their gametes the genome that was present in two copies; i.e. the L genome was inherited from LLR triploids and the R genome from RRL triploids. Their findings confirm the principal inheritance patterns of P. esculentus, which have been known for more than three decades (e.g. Uzzell et al. 1975; Günther et al. 1979; Uzzell et al. 1980; Vinogradov et al. 1990). Transmission of two L genomes by LLR males, which was observed in the Slovakian population Šajdíkove (Mikulíček and Kotlík 2001, Pruvost et al. 2013), can be considered a rare exception. It is not certain, however, that all LLR males of this population produce only LL sperm (nine of 14 LLR males transmitted only LL gametes, but five such males produced no progeny) nor is it certain that LR males in this population transmit only the R genome (only eight F1 individuals from two crosses involving a single male could be genotyped). That no P. lessonae individuals were found in a sample of 169 individuals from this population (Mikulíček and Kotlík 2001; Provost et al. 2013) suggests that the LR individuals in this population originate from LR x LLR and/or LR x LR crosses; as mentioned by Pruvost et al., both LLR males and occasionally LR males and females are able to produce L gametes (Binkert et al. 1982; Günther 1983). Only a few crosses in which either the diploid or the triploid parent produced L gametes would be sufficient for the persistence of such all-hybrid populations; in a relatively large Polish esculentus population comprising 300-400 females, for example, less than 1% of the eggs laid transformed to tadpoles (Berger 1988). Thus, even a high number of artificial crossing experiments does not guarantee that rare inheritance patterns, which may be critical to population persistence, are discovered. Based on the 10 genetic markers that they used, Pruvost et al. suggested that the two L genomes transmitted by LLR males from Šajdíkove are identical. Identity of markers does not, however, necessarily mean identity of genomes. Mikulíček and Kotlík (2001) investigated one LLR male from this population electrophoretically and found two distinct lessonae-specific alleles at the ldh-1 locus, which is evidence that the L genomes of this male differed from each other. On the other hand, it is not certain that in all cases “triploid hybrids recombine the genome they have in double dose” (Pruvost et al. 2013), although evidence for recombination between the double genomes in triploids comes from enzyme loci (Günther et al. 1979) and microsatellites (Christiansen and Reyer 2009). Because the few polymorphic markers available until recently do not allow distinguishing between the double genomes in many triploids, it cannot be said whether recombination between the two L or the two R genomes has taken place in such individuals. Biases in sex ratio of the progeny from crosses in which triploid individuals were involved (Günther et al. 1979; Berger and Günther 1988; 1991-1992), putative recombination between the L and the R genome in triploids (e.g. Plötner and Klinkhardt 1992; Christiansen et al. 2005), the occasional production of LL and LR eggs by some triploid (LLR) females (Christiansen et al. 2005; Christiansen 2009), the rare production of R or LL sperm by LLR males (Brychta and Tunner 1994; Christiansen et al. 2005), incomplete elimination of the R genome in some spermatogonia (Vinogradov et al. 1990), and chromosomal aberrations in meiosis of triploid males (Günther 1975) reflect the huge complexity of inheritance in triploid P. esculentus. P. esculentus may represent a useful model for hybrid speciation. It is not surprising, however, that hybrid forms in early stages of speciation exhibit a plethora of genetic disturbances and irregularities (Dobzhansky-Muller incompatibilities; e.g. reviewed by Landry et al. 2007; Maheshwari and Barbasch 2011) especially in gametogenesis and embryonic development expressed as fertility disorders in adults (Günther 1973), abnormal cleavage of eggs, malformations in larvae, and high mortality during larval development (e. g. Christiansen et al. 2005; reviewed by Ogielska 2009). The observed differences in the inheritance patterns of triploid water frog hybrids may thus be interpreted as a simple result of selection-mediated interactions between specific genomic features and spatial-environmental conditions of different evolutionary lineages representing a monophyletic group; at present there is no character-based evidence that triploid frogs are of polyphyletic origin as proposed by Pruvost et al. More genomic data are required to answer this and many other questions concerning the genetics and evolutionary history of western Palearctic water frogs.

List of references can be downloaded from here

http://www.peterbeerli.com/papers/misc/Reply_Pruvost_et_al_2013.pdf

1 Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Invalidenstraße 43, 10115 Berlin. Germany. Email: joerg.ploetner@mfn-berlin.de

2 Field Station Dätwil, 8452 Adlikon, Hauptstrasse 2, Switzerland

3 Florida State University, Department of Scientific Computing, Tallahassee, FL 32306-4120, USA

4 Academy of Natural Sciences, Laboratory for Molecular Systematics and Ecology, 1900 B. F. Parkway, PA 19103 Philadelphia, USA

On 2014 Jan 29, Peter Beerli commented:

Inheritance patterns in diploid and triploid water frog hybrids (Pelophylax esculentus) – a comment on Pruvost et al.

Jörg Plötner^1, Gaston-Denis Guex^2, Peter Beerli^3, and Thomas Uzzell⁴

(Addresses at the end)

Pruvost et al. (2013) described the gamete production and ploidy of water frogs from five populations in Germany, Poland, and Slovakia. Two populations were composed of P. lessonae (genotype LL), P. ridibundus (RR) and their hybridogenetic hybrid P. esculentus (LR, LLR, RRL) whereas three populations consisted of only diploid LR individuals and triploids (LLR, RRL). Based on crossing experiments involving 64 P. esculentus and the analysis of microsatellites, the authors found that diploid males (genotype LR) produced haploid gametes with a ridibundus (R) genome whereas LR females usually produced diploid eggs containing both parental genomes (LR gametes). Moreover, most of the triploid individuals transmitted to their gametes the genome that was present in two copies; i.e. the L genome was inherited from LLR triploids and the R genome from RRL triploids. Their findings confirm the principal inheritance patterns of P. esculentus, which have been known for more than three decades (e.g. Uzzell et al. 1975; Günther et al. 1979; Uzzell et al. 1980; Vinogradov et al. 1990). Transmission of two L genomes by LLR males, which was observed in the Slovakian population Šajdíkove (Mikulíček and Kotlík 2001, Pruvost et al. 2013), can be considered a rare exception. It is not certain, however, that all LLR males of this population produce only LL sperm (nine of 14 LLR males transmitted only LL gametes, but five such males produced no progeny) nor is it certain that LR males in this population transmit only the R genome (only eight F1 individuals from two crosses involving a single male could be genotyped). That no P. lessonae individuals were found in a sample of 169 individuals from this population (Mikulíček and Kotlík 2001; Provost et al. 2013) suggests that the LR individuals in this population originate from LR x LLR and/or LR x LR crosses; as mentioned by Pruvost et al., both LLR males and occasionally LR males and females are able to produce L gametes (Binkert et al. 1982; Günther 1983). Only a few crosses in which either the diploid or the triploid parent produced L gametes would be sufficient for the persistence of such all-hybrid populations; in a relatively large Polish esculentus population comprising 300-400 females, for example, less than 1% of the eggs laid transformed to tadpoles (Berger 1988). Thus, even a high number of artificial crossing experiments does not guarantee that rare inheritance patterns, which may be critical to population persistence, are discovered. Based on the 10 genetic markers that they used, Pruvost et al. suggested that the two L genomes transmitted by LLR males from Šajdíkove are identical. Identity of markers does not, however, necessarily mean identity of genomes. Mikulíček and Kotlík (2001) investigated one LLR male from this population electrophoretically and found two distinct lessonae-specific alleles at the ldh-1 locus, which is evidence that the L genomes of this male differed from each other. On the other hand, it is not certain that in all cases “triploid hybrids recombine the genome they have in double dose” (Pruvost et al. 2013), although evidence for recombination between the double genomes in triploids comes from enzyme loci (Günther et al. 1979) and microsatellites (Christiansen and Reyer 2009). Because the few polymorphic markers available until recently do not allow distinguishing between the double genomes in many triploids, it cannot be said whether recombination between the two L or the two R genomes has taken place in such individuals. Biases in sex ratio of the progeny from crosses in which triploid individuals were involved (Günther et al. 1979; Berger and Günther 1988; 1991-1992), putative recombination between the L and the R genome in triploids (e.g. Plötner and Klinkhardt 1992; Christiansen et al. 2005), the occasional production of LL and LR eggs by some triploid (LLR) females (Christiansen et al. 2005; Christiansen 2009), the rare production of R or LL sperm by LLR males (Brychta and Tunner 1994; Christiansen et al. 2005), incomplete elimination of the R genome in some spermatogonia (Vinogradov et al. 1990), and chromosomal aberrations in meiosis of triploid males (Günther 1975) reflect the huge complexity of inheritance in triploid P. esculentus. P. esculentus may represent a useful model for hybrid speciation. It is not surprising, however, that hybrid forms in early stages of speciation exhibit a plethora of genetic disturbances and irregularities (Dobzhansky-Muller incompatibilities; e.g. reviewed by Landry et al. 2007; Maheshwari and Barbasch 2011) especially in gametogenesis and embryonic development expressed as fertility disorders in adults (Günther 1973), abnormal cleavage of eggs, malformations in larvae, and high mortality during larval development (e. g. Christiansen et al. 2005; reviewed by Ogielska 2009). The observed differences in the inheritance patterns of triploid water frog hybrids may thus be interpreted as a simple result of selection-mediated interactions between specific genomic features and spatial-environmental conditions of different evolutionary lineages representing a monophyletic group; at present there is no character-based evidence that triploid frogs are of polyphyletic origin as proposed by Pruvost et al. More genomic data are required to answer this and many other questions concerning the genetics and evolutionary history of western Palearctic water frogs.

List of references can be downloaded from here

http://www.peterbeerli.com/papers/misc/Reply_Pruvost_et_al_2013.pdf

1 Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Invalidenstraße 43, 10115 Berlin. Germany. Email: joerg.ploetner@mfn-berlin.de

2 Field Station Dätwil, 8452 Adlikon, Hauptstrasse 2, Switzerland

3 Florida State University, Department of Scientific Computing, Tallahassee, FL 32306-4120, USA

4 Academy of Natural Sciences, Laboratory for Molecular Systematics and Ecology, 1900 B. F. Parkway, PA 19103 Philadelphia, USA