Variability of NOR patterns in European water frogs of different genome composition and ploidy level.

Anna Zaleśna,Mariusz Rybacki,Maria Ogielska,Maria Florek

doi:10.3897/compcytogen.v11i2.10804

Abstract

We studied water frogs from a complex composed of two species: Pelophylax lessonae (Camerano, 1882) (genome LL, 2n = 26) and P. ridibundus (Pallas, 1771) (RR, 2 = 26), and their natural hybrid P. esculentus (Fitzinger, 1843) of various ploidy and genome composition (RL, 2n = 26, and RRL or RLL, 3n = 39). Tetraploids RRLL were found (4n = 52) in juveniles. We applied cytogenetic techniques: AgNO3, chromomycin A3, PI and fluorescent in situ hybridization with a 28S rDNA probe. Results obtained by silver staining corresponded well with those stained with CMA3, PI and FISH. As a rule, NORs are situated on chromosomes 10. The number of Ag-NORs visible on metaphase plates was the same as the number of Ag-nucleoli present in interphase nuclei of the same individual. In all analyzed metaphases, NORs exhibited variations in size after AgNO3 and CMA3 stainings. Sixty-six individuals (out of 407 analyzed) were polymorphic for the localization and number of NORs. Fifty-one diploids had NORs only on one chromosome of pair 10. Three triploids (LLR and RRL) displayed two NORs, and two other triploid RRL individuals displayed one, instead of expected three NORs. In ten individuals extra NORs were detected on chromosomes other than 10 (chromosomes 2 and 9).

Highlights

Nucleolus organizer regions (NORs) are sites of nucleoli formation owing to the presence of genes coding for 18S rRNA, 5.8S rRNA and 28S rRNA
After AgNO3 staining, one NOR per a haploid set was identified in secondary constrictions on long arms of chromosomes 10 in 341 individuals (83.8%), and in these individuals the number of NORs reflected the ploidy level
After actinomycin D (AMD)/DAPI staining we discovered that in the case of the RRL females the lack of NORs was inherited together with the haploid set of the ridibundus chromosomes, whereas in the case of the LLR female it was inherited with the haploid set of the lessonae chromosomes

Summary

Introduction

Nucleolus organizer regions (NORs) are sites of nucleoli formation owing to the presence of genes (rDNA) coding for 18S rRNA, 5.8S rRNA and 28S rRNA They are the only genes that can be recognized in genomes on the basis of chromosome structure and thereby are useful as cytogenetic markers. The localization of NORs is conservative, i.e. they are almost always located intercalary or proximally to the centromeres or close to telomeres or related to other regions rich in heterochromatin (Iizuka et al 2013) Exceptions to this rule may suggest chromosomal rearrangements that have occurred in NOR-carrying chromosome segments during evolution (Schmid 1982). Similar results were described in Physalaemu olfersii (Silva et al 2000), Hypsiboas (Carvalho et al 2014), Physalaemus cuvieri (Quinderé et al 2009), and in the Jefferson salamanders Ambystoma jeffersonianum (Bi et al 2009)

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