Cross-Specific Mortality and Differential Occurrence of Aberrant Phenotypes in Tadpoles of the Pelophylax Kl

SALAMANDRA 55(2) 127–130 15 May 2019 CorrespondenceISSN 0036–3375 Correspondence Cross-specific mortality and differential occurrence of aberrant phenotypes in tadpoles of the Pelophylax kl. esculentus hemiclone assortment Alessandro Bolis, Daniele Pellitteri-Rosa & Adriana Bellati Department of Earth and Environmental Sciences, University of Pavia, Via Ferrata 9, 27100 Pavia, Italy Corresponding author: Adriana Bellati, e-mail: [email protected] Manuscript received: 23 September 2018 Accepted: 14 January 2019 by Arne Schulze Morphological malformations are one of the factors con- gano et al. 1997), thus masking deleterious recessive alle- tributing to amphibian declines worldwide. There is grow- les carried by the non-recombinant R genome via perma- ing evidence that these are boosted by altered ecological nent heterozygosity with the host parental species (Berger conditions like increased UV-B radiation, parasite infec- 1967, Graf & Müller 1979, Graf & Polls-Pelaz 1989, tions, and pollution (e.g., Blaustein et al. 1997, Johnson Vorburger 2001). et al. 2001). Recent advances in molecular techniques to- As a result, natural mating between two hybrids in L-E day facilitate more in-depth studies of inbreeding, i.e., the systems typically leads to non-viable RR tadpoles (individ- union of consanguineous gametes that can lead to a signifi- uals that carry two clonally transmitted ridibundus genom- cant decrease in fitness by favouring the expression of reces- es), as is evidenced by the absence of adult, and in most sive deleterious alleles in homozygotes (Keller & Waller cases of metamorphosed juvenile P. ridibundus in L-E sys- 2002, Charlesworth & Willis 2009). Such reduction in tems throughout their range, and the production of non- genetic diversity can similarly promote the occurrence of viable offspring, with high mortality rates during the early morphological malformations in natural populations of tadpole developmental stages (97%; Berger 1967, Graf & many different taxa like fishesAfonso ( et al. 2000), mam- Polls-Pelaz 1989). Genomic interactions between differ- mals (Mansfield & Land 2002), reptiles (Olsson et al. ent hemiclones instead lead to less negative outcomes (in 1996), and amphibians (Williams et al. 2008). terms of larval development and metamorphosis) accord- Water frogs of the genus Pelophylax Fitzinger, 1843 ing to the cross-specific expression of recessive deleterious (Anura, Ranidae) offer a unique opportunity for study- mutations (Guex et al. 2002). ing relationships between the onset of body malformations Here, we report the occurrence of a higher mortality and inbreeding. Some species indeed exhibit a peculiar re- rate and abnormal development, i.e., the incidence of an productive mode called hybridogenesis (Schultz 1969) in external morphological malformation in tadpoles gener- which two different parental species crossbreed to produce ated by crossings between identical P. kl. esculentus same a fertile hybrid (hemiclone) that will carry the genomes of hemiclones from the same population in comparison with both parental species in its somatic cells and produce hap- offspring resulting from crossbreeding between different loid gametes that only carry one non-recombinant paren- hemiclones sampled from different populations. tal genome. In eastern and central Europe, the Pelophylax Adult frogs were caught in the field from two differ- esculentus complex (Frost et al. 2006) occurs, which in- ent populations in northwestern Italy (Lombardy). Sam- volves two parental species: Pelophylax lessonae (Camer- pling sites were selected according to previous knowledge ano, 1882) and Pelophylax ridibundus (Pallas, 1771), and on the distribution of native taxa, to avoid collecting alien their fertile hybrid P. klepton esculentus (Linnaeus, 1758) individuals or their hybrids (Bellati et al. 2012). Specifi- (Berger 1988). Prior to meiosis, P. kl. esculentus eliminates cally, sampling was carried out at a lowland site (S1, Ber- the P. lessonae genome (Borkin et al. 1979, Tunner & Hep- nate Ticino: 45.47 N, 8.80 E, 120 m a.s.l.) and a highland pich 1981), transmitting the P. ridibundus genome clonally. site (S2, Armelio Mt.: 44.73 N, 9.45 E, 1025 m a.s.l.) during In northern Italy, P. kl. esculentus naturally coexists with the peak of the reproductive season, which may last from P. lessonae in the so-called L-E system (Uzzel et al. 1976). late March through May depending on altitude; Lanza et Here, the fertile hybrid mates with the parental species al. 2007). Frogs were collected by hand or net using torch- to maintain the hybrid condition in each generation (Pa- lights at night, and assigned to native taxa based on their © 2019 Deutsche Gesellschaft für Herpetologie und Terrarienkunde e.V. (DGHT), Mannheim, Germany Available at http://www.salamandra-journal.com 127 Correspondence morphologies. In the field, specimens were measured with ing to their nuclear genotypes, i.e., all loci were success- digital callipers (± 1.0 mm), and subadults (SVL, snout– fully amplified but only those assumed to be non-selective vent length, < 60 mm; Lanza et al. 2007) were immedi- for the L- or R-genomes turned out to be heterozygotes. ately released at their capture sites. More precisely, genotypes from S1 (from male ABZC03417 Adult frogs, two males (ABZC03417 from population and female ABZC03430) exhibited perfect allele-sharing S1 and ABZC03311 from population S2) and one female at nuclear loci, attesting both individuals sharing the same (ABZC03430 from population S1) were transferred to the hemiclone. In contrast, alleles of male ABZC03311 from laboratory and housed in plastic tanks filled with matured population S2 were nearly entirely private, suggesting it tap water, outfitted with bricks and leaves as shelters, exter- was representative of a distinct hemiclone. nal filters (EHEIM ecco pro 200, EHEIM, Germany), and a Breeder specimens were crossed in vitro following UV-B lamp (2.0%, Sera, Germany) each. The frogs were fed Berger’s protocol (Berger et al. 1994) with minor modi- daily with live crickets (Acheta domestica) and meal beetles fications to optimise usage of clutches and sperm suspen- (Tenebrio molitor), fortified with a calcium and vitamin 3D sions (Bellati et al. unpubl. data). Before manipulations, supplement (ZooMed, California, USA). Prior to their be- males were euthanised by immersion in a MS-222 water ing accommodated thus, we toe-clipped the individuals to bath solution (0.5g/l). Mature eggs were obtained by gently individualize them and collect biological samples for sub- pressing the female’s venter. sequent molecular analysis. Crossings were performed on 4 July 2017, and hatching Genomic DNA was extracted using a commercial kit occurred one week later, on 10 July 2017 (determined when (Sigma-Aldrich, Saint Louis, USA) following manufactur- 50% of the tadpoles had hatched from their eggs). A total er’s instructions. Because members of this genus are diffi- of 150 viable tadpoles were selected from each crossing, di- cult to identify morphologically and to confirm their phe- vided into three replicates (50 tadpoles each), and raised notypic assignment, we firstly amplified the mitochondrial indoors in plastic tanks filled with 10 litres of matured gene, encoding for NADH dehydrogenase subunit 3 (ND3, tap water under natural daylight conditions. Temperature 340 base pairs in length), which discriminates between dis- was checked daily using a digital thermometer (± 0.1°C, tinct Pelophylax taxa (Plötner et al. 2008). Amplifications Greisinger Electronic, Germany) to ensure homogene- were set up using 0.5 U of HotStart Taq DNA Polymerase ity between tanks. Tadpoles were monitored daily to as- (biotechrabbit GmbH, Hennigsdorf, Germany) and pub- sess hatching dates and developmental stages according lished primer pairs (Plötner et al. 2008). PCR products to Gosner (1960). After reaching stage 28, tadpoles were were sequenced externally (Eurofins Genomics, Ebersberg, fed with dry rabbit food, and monitored for 80 days, which Germany). Raw electropherograms were visually checked corresponds to the period required for native water frog to exclude the presence of double peaks and translated into tadpoles to reach metamorphosis according to literature amino acids using Geneious v11 (Kearse et al. 2012) to de- (Lanza et al. 2007). Both malformation and death rates tect possible premature stop codons suggesting the pres- were recorded 42 days after hatching date (corresponding ence of pseudogenes. Aligned sequences were compared to Gosner stage 30 in our experimental offspring) and at in GenBank (https://www.ncbi.nlm.nih.gov/) using the the end of the observation period on 21 September 2017 BLAST (Basic Local Alignment Search Tool, https://blast. (corresponding to Gosner stage 36). Under our standard- ncbi.nlm.nih.gov/Blast.cgi) algorithm to infer mitochon- ized housing conditions, none of the individuals reached drial species assignment. metamorphosis. As P. kl. esculentus carries a lessonae-type mtDNA in The level of mortality during the monitoring period Italian populations of the L-E system, we also screened 9 was investigated using the two-sample Student’s t-test. We codominant microsatellite markers that, according to the observed a significantly higher mortality rate in tadpoles literature (Garner et al. 2000, Zeisset et al. 2000, Ario- generated by the crossing between individuals from the li 2007, Christiansen & Reyer 2009), are known to am- same population (Crossing 1) than those by Crossing 2 in plify

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