Fig 1.
Typical lampbrush chromosome sets from oocytes of triploid hybrid frogs with RRL and LLR genotypes and diploid hybrid frog with RL genotype.
Full lampbrush chromosome sets from oocytes of triploid hybrid frog with RRL (a,a`) and LLR (c,c`) genotypes and diploid hybrid frog with RL genotype (b,b`). Chromosome sets are represented by 13 bivalents, which have distribution of marker structures corresponding to P. ridibundus (a,a`,b,b`) or P. lessonae (c,c`) lampbrush chromosomes. Letter symbols indicate alphabetic numbering of all lampbrush chromosomes; italic type shows correspondence of identified chromosomes to genotype of parental species: r—to P. ridibundus, l—to P. lessonae. Chromosomes were counterstained with DAPI (a,b,c). Corresponding phase-contrast micrographs are shown (a`,b`,c`). Scale bars = 50 μm.
Fig 2.
Interstitial (TTAGGG)n repeat sites mapping allows to identify parental chromosomes in oocytes of hybrid frogs.
(a-c) FISH mapping of (TTAGGG)n repeat on metaphase chromosomes of P. lessonae (a, a`), P. ridibundus (b), and diploid P. esculentus (c). One or two interstitial (TTAGGG)n repeat sites distinguish parental NOR-bearing chromosomes H (arrows). Asterisks indicate enlarged fragment with two NOR-bearing chromosomes of P. lessonae. Arrows indicate interstitial (TTAGGG)n repeat sites. (d1–f1`) Lampbrush chromosomes from oocytes of triploid hybrid frogs with RRL (d1–d6`) and LLR (e1–f1`) genotypes. FISH mapping of (TTAGGG)n repeat revealed lampbrush chromosome H corresponding to P. ridibundus (d6) or P. lessonae (e1) LBC H. Interstitial (TTAGGG)n repeat sites are shown by square brackets. Lampbrush chromosomes corresponding to P. ridibundus LBC F (d1,d1`), G (d2,d2`), D (d3,d3`), I (d4,d4`), B (d5,d5`), and P. lessonae LBC B (e2,b2`), F (e3,b3`), L (f1,f1`) are shown. Chromosomes on micrographs (d1–d6`) were taken from the full lampbrush chromosome set represented on Fig 1a,a`. Chromosomes on micrographs (e1–e3`) were taken from the from the full lampbrush chromosome set represented on Fig 1c,c`. Various marker structures are shown by arrows. Chromosomes were counterstained with DAPI. Corresponding phase-contrast micrographs are shown (d1`,d2`,d3`,d4`,d5`,d6`,e1`,e2`,e3`,f1`). Arrowheads indicate centromeres. Scale bars = 10 μm for all panels except a`, where scale bar = 2 μm.
Fig 3.
Suggested mechanisms of oogenesis typical for triploid hybrid frogs with RRL genotype and diploid hybrid frogs.
(a) During oogenesis of the majority of triploid hybrids with RRL genotype from studied population systems of R-E type, L genome (blue) was eliminated while two remaining R genomes (brown and orange) without endoreplication formed 13 bivalents. (b) In oogenesis of the majority of diploid hybrids with RL genotype from studied population systems of R-E type, L genome (blue) was eliminated and the remaining R genome (orange) was endoreplicated to form 13 bivalents.
Fig 4.
Unusual lampbrush chromosome sets from oocytes of triploid hybrid frogs with RRL genotype and two diploid hybrid frogs.
(a,a`,b,b`) Lampbrush chromosome sets from oocytes of some triploid hybrid frogs with RRL genotype represented by 39 bivalents (a,a`) and 39 univalents (b,b`), with 26 bi- or univalents corresponding to P. ridibundus lampbrush chromosomes and 13 bi- or univalents corresponding to P. lessonae lampbrush chromosomes. (c,c`) Lampbrush chromosome set from oocyte of one diploid hybrid frog represented by 26 univalents. Some univalents are similar to P. ridibundus lampbrush chromosomes, while other univalents are similar to P. lessonae lampbrush chromosomes. (d,d`) Lampbrush chromosome set from oocyte of one diploid hybrid frog represented by 26 bivalents corresponding to P. ridibundus lampbrush chromosomes. Letter symbols indicate alphabetic numbering of all lampbrush chromosomes; italic type shows correspondence of identified chromosomes to genotype of parental species: r—to P. ridibundus, l—to P. lessonae. Chromosomes were counterstained with DAPI (a,b,c,d). Corresponding phase-contrast micrographs are shown (a`,b`,c`,d`). Scale bars = 50 μm.
Fig 5.
Suggested additional mechanisms of oogenesis in two triploid frogs with RRL genotype and one diploid hybrid frog.
(a) During oogenesis of one triploid frog with RRL genotype neither elimination nor endoreplication occurred to form oocytes with 39 univalents (at the top), endoreplication of all genomes took place to form oocytes with 39 bivalents (in the middle), individual chromosomes from L genome (blue) were lost to form oocytes with aneuploid chromosomal sets (at the bottom). (b) During oogenesis of another triploid frog with RRL genotype elimination of L genome (blue) occurred to form oocytes with 13 bivalents (at the top), premeiotic elimination and endoreplication were absent to form oocytes with 39 univalents (in the middle), endoreplication of all genomes took place to form oocytes with 39 bivalents (at the bottom). (c) During oogenesis of one diploid hybrid frog L genome (blue) was eliminated in all observed oocytes. One round of R genome (orange) endoreplication occurred but bivalents formation was incomplete to form oocytes with both univalents and bivalents (at the top). Two rounds of endoreplication of R genome took place to form oocytes 26 bivalents (in the middle). One round of R genome endoreplication occurred but bivalents could not form that led to formation of oocytes with 26 univalents (at the bottom).