Cytological maps of lampbrush chromosomes of European water frogs (Pelophylax esculentuscomplex) from the Eastern Ukraine

Abstract

BackgroundHybridogenesis (hemiclonal inheritance) is a kind of clonal reproduction in which hybrids between parental species are reproduced by crossing with one of the parental species. European water frogs (Pelophylax esculentus complex) represent an appropriate model for studying interspecies hybridization, processes of hemiclonal inheritance and polyploidization. P. esculentus complex consists of two parental species, P. ridibundus (the lake frog) and P. lessonae (the pool frog), and their hybridogenetic hybrid – P. esculentus (the edible frog). Parental and hybrid frogs can reproduce syntopically and form hemiclonal population systems. For studying mechanisms underlying the maintenance of water frog population systems it is required to characterize the karyotypes transmitted in gametes of parental and different hybrid animals of both sexes.ResultsIn order to obtain an instrument for characterization of oocyte karyotypes in hybrid female frogs, we constructed cytological maps of lampbrush chromosomes from oocytes of both parental species originating in Eastern Ukraine. We further identified certain molecular components of chromosomal marker structures and mapped coilin-rich spheres and granules, chromosome associated nucleoli and special loops accumulating splicing factors. We recorded the dissimilarities between P. ridibundus and P. lessonae lampbrush chromosomes in the length of orthologous chromosomes, number and location of marker structures and interstitial (TTAGGG)n-repeat sites as well as activity of nucleolus organizer. Satellite repeat RrS1 was mapped in centromere regions of lampbrush chromosomes of the both species. Additionally, we discovered transcripts of RrS1 repeat in oocytes of P. ridibundus and P. lessonae. Moreover, G-rich transcripts of telomere repeat were revealed in association with terminal regions of P. ridibundus and P. lessonae lampbrush chromosomes.ConclusionsThe constructed cytological maps of lampbrush chromosomes of P. ridibundus and P. lessonae provide basis to define the type of genome transmitted within individual oocytes of P. esculentus females with different ploidy and from various population systems.

Transcription of highly repetitive tandemly organized DNA in amphibians and birds: A historical overview and modern concepts

“…[46][47][48] Recently, telomeric repeat transcripts were revealed on the lampbrush chromosomes of frogs (P. ridibundus and P. lessonae). 40 Strand-specific pattern of the telomere repeat transcription is identical in birds, amphibians and mammals. In mammalian cells transcription starts from the subtelomeric regions on several chromosomal ends along with the C-rich strand of the repeat.…”

“…84,85 It should be noted that it is quite difficult to obtain mammalian female gametes preparations, especially preparations of oocytes at the first meiotic division, which limited the studies of the biological role of TERRA RNA and TERT in gametogenesis. Transcription of telomere repeat that was demonstrated at the termini of lampbrush chromosomes 40,44,45 makes amphibian and avian oocytes an attractive model for elucidation of TERRA RNA functions and biogenesis.…”

“…1B and C). 40 Satellite 1 of the Notophthalmus viridescens with (peri-)centromeric and 2 non-centromeric sites is transcriptionally active on the lampbrush chromosomes. 36 One of the 2 satellite DNA sequences (sat G family), called pTvml, with presumably pericentromeric localization is transcribed on the lampbrush chromosomes of T. vulgaris meridionalis.…”

“…1B and C). 40 The direction of satellite DNA transcription on chromosomal lateral loops as well as the transcribed strands can vary in different species. For instance, X1-741 repeat of X. laevis, satellites 1 and 2 of the N. viridescens and satellite DNA of T. cristatus carnifex are transcribed from both strands.…”

“…[46][47][48][49] Telomere repeat transcripts on the anuran amphibian lampbrush chromosomes form small caps at the ends of half-bivalents and were detected with (TAACCC) 5 FISH probe. 40 Mammalian TERRA RNA is heterogeneous in length and is transcribed by RNA polymerase II. 46,47 In the somatic cells, TERRA RNA forms a complex with proteins and accumulates into the nuclear foci known as TERF (TElomeric Repeat-binding Factor) foci.…”