Research Article |
Corresponding author: Sergey Matveevsky ( sergey8585@mail.ru ) Academic editor: Jan Zima
© 2015 Sergey Matveevsky, Irina Bakloushinskaya, Valentina Tambovtseva, Svetlana Romanenko, Oxana Kolomiets.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Matveevsky S, Bakloushinskaya I, Tambovtseva V, Romanenko S, Kolomiets O (2015) Analysis of meiotic chromosome structure and behavior in Robertsonian heterozygotes of Ellobius tancrei (Rodentia, Cricetidae): a case of monobrachial homology. Comparative Cytogenetics 9(4): 691-706. https://doi.org/10.3897/CompCytogen.v9i4.5674
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Synaptonemal complex (SC) chains were revealed in semisterile intraspecific F1 hybrids of Ellobius tancrei Blasius, 1884 (2n = 49, NF=56 and 2n=50, NF=56), heterozygous for Robertsonian (Rb) translocations. Chains were formed by Rb submetacentrics with monobrachial homology. Chromosome synapsis in spermatocytes of these hybrids was disturbed, apparently because of the problematic release of the chromosomes from the SC chains. These hybrids suffer from low fertility, and our data support the opinion that this is because a formation of Rb metacentrics with monobrachial homology within different races of the same species might be an initial event for the divergence of chromosomal forms.
Meiosis, synaptonemal complex, SC multivalent, hybrid, fertility
Many authors have described the significant impact of chromosomal rearrangements as reproductive barriers in the speciation process (
Heterozygosity for the Rb translocations may seriously affect the segregation of chromosomes during meiosis. In heterozygotes, the first division of meiosis will take place without complications only if both acrocentrics, homologous to Rb metacentric arms, move into one pole, whereas the metacentric moves to the opposite pole of the nucleus. Otherwise, if the Rb metacentric and one of the homologous acrocentrics go to the same pole of the nucleus, an inadequate segregation of chromosomal arms will take place and aneuploid gametes will develop (
A study of the structure and behavior of the synaptonemal complex (SC) is crucial for understanding chromosome synapsis, particularly in heterozygous animals. SC is a skeleton of meiotic bivalents, which form between homologous chromosomes in prophase I of meiosis. The SC analysis allows us to trace all details of the formation of its axial elements, from the beginning of pairing and synapsis at the zygotene, especially synapsis and its correction in heterozygotes for chromosome rearrangements at the pachytene and desynapsis dynamics at diplotene.
The eastern mole vole Ellobius tancrei (Cricetidae, Rodentia) is an interesting example of the wide variability of chromosome numbers (from 2n = 30 to 2n = 54 with a stable NF = 56) (
Applying comparative chromosome painting (Zoo-FISH) helped to prove the occurrence of non-homologous Rb translocations in different populations of E. tancrei (
The specimens studied by us were obtained from laboratory colonies. Hybrid animals were bred in the laboratory from crosses between animals, homozygous for the diagnostic fusions. Animals were kept under standard conditions with free access to food at the facility of the Koltzov Institute of Developmental Biology RAS. Animals were treated according established international protocols, such as the Guidelines for Humane Endpoints for Animals Used in Biomedical Research, and Regulations for Laboratory Practice in Russian Federation, and under the supervision of the Ethics Committee for Animal Research of the Koltzov Institute of Developmental Biology.
As a control we study SCs of two adult males with 2n=54, obtained from two points in Tajikistan: near Miskinobod 38° 39.78’ N; 69° 33.29’ E, 1780 m above sea level, and Panchkotan valley, the left bank of the Sorbo River 38° 45.27’ N; 69° 17.6’ E, 1265 m above sea level. Parental forms for F1 hybrids originated from the right bank of the Surkhob River, close to the airport Garm, 39° 0.28’ N; 70° 17.77’ E, 1310 m above sea level (2n=48), and from the opposite bank of the Surkhob River near the Voydara settlement 38° 58.9’ N; 70° 14.71’ E, 1440 m above sea level (2n = 50). Eight F1 hybrids (2n = 49), five newborn females and three adult males (not younger than 1 year), were investigated. For the second intraspecific crossing we use animals from Voidara (2n=50) and from the Varzob Valley, near the Khodzha-Obi-Garm settlement 38° 53.53’ N; 68° 46.52’ E, 2020 m above sea level (2n = 50). Five adult males of F1 hybrids (2n = 50) were investigated.
Chromosomes from bone marrow (
The suspensions of oocytes and spermatocytes were prepared by the method described by
The slides were stained with 50% AgNO3 in a humid chamber at 56 °C for 3 hours after which they were washed four times in distilled water and air dried. The stained slides were observed by light microscope and suitably spread cells were selected. Plastic circles were cut out with a diamond tap and transferred onto grids. The slides were then examined under a JEM 100B electron microscope.
The slides were washed in PBS. Whole mount SCs were blocked with HB (holding buffer: PBS, 0.3% BSA, 0.005% Triton X-100). The slides were incubated overnight at 4 °C with rabbit polyclonal antibodies against the human lateral element protein SCP3 (Abcam, 15093Ab, UK Cambridge, UK) diluted to a concentration of 1:200 in ADB (Antibody Dilution Buffer: PBS, 3% BSA, 0.05% Triton X-100), human anti-centromere antibodies ACA, 1:200 (Antibody Incorporated, California, USA), and mouse monoclonal antibodies to human mismatch repair protein MLH1, 1:50 (Abcam, Cambridge, UK). The slides were washed in PBS and incubated with goat anti-rabbit Alexa Fluore 488 conjugated antibodies (1:800, Abcam, Cambridge, UK) and goat anti-human Alexa Fluore 546 conjugated antibodies (1:800) at 37 °C for 60 min. The slides were washed with PBS, rinsed briefly with distilled water, dried and mounted in Vectashield with DAPI (Vector Laboratories).
Meiotic cellular suspensions from testes for post-pachytene stages analysis were prepared using the air-drying method (
The slides were analyzed with an Axioimager D1 microscope CHROMA filter sets (Carl Zeiss, Jena, Germany) equipped with a Axiocam HRm CCD camera (Carl Zeiss), and image-processing AxioVision Release 4.6.3. software (Carl Zeiss, Germany). Images were processed using Adobe Photoshop CS3 Extended. Measurements of autosomal bivalents and their ranking in each cell were made in order to determine relative lengths (MicroMeasure 3.3, Colorado, USA) using the STATISTICA 8.0 software (StatSoft, Tulsa, OK, USA).
Karyotypes of parental forms from opposite banks of the Surkhob River were published earlier (
Chromosome synapsis in pachytene spermatocytes of F1 hybrid E. tancrei (2n = 49, NF = 56). a The scheme reflects a prognosis for chromosome synapsis in prophase I of meiosis [M1=Rb(3.18), M2=Rb(2.18), and M3=Rb(2.11)] b Closed SC pentavalent. Immunostaining with antibodies to SC protein 3 (green) and to the centromere ACA (red) c The scheme of chromosome synapsis in the structure of SC pentavalent (see Fig. 1b). Black dots mark centromere positions d Electron micrograph of spread spermatocyte from F1 hybrid. Closed SC pentavalent is formed from three metacentrics with monobrachial homology (M1, M2, and M3) and two acrocentrics (A1, A2). The arrow shows the fragment of SC between the short arms of homologous acrocentrics. Gaps are marked with asterisks. A sex bivalent (XX) does not associate with the multivalent. Nb – nucleolus-like bodye The scheme of chromosome synapsis in the structure of SC pentavalent (see Fig. 1d) f A complex case of the association of sex bivalent (XX) g The scheme of the association of sex bivalent (XX) with an autosome (see Fig. 1f) h An association of sex bivalent (XX) i The scheme of the association of sex bivalent (XX) with an autosome (see Fig. 1h). Scale bars: 1 μm (b, d, i); 2 μm (e, f, h).
We identified the Rb translocations in the form with 2n = 50, “Khodza Obi-Garm”, which had four Rb metacentrics [2Rb(4.12) and, 2Rb(9.13)]. F1 hybrids of crossings “Khodza Obi-Garm” with 2n = 50, “Voidara” had four different Rb metacentrics, two of them with MBH [Rb(5.9) and Rb(9.13)] (Fig.
As a control, we used E. tancrei with 2n=54, which is typical for the species. Their fertility was 2.37±0.22 (71 litters, 168 pups). In crossing type I (48 × 50), 24 litters and 62 hybrids F1 were obtained. Litter size was 2.58 ± 1.02. In inbred crosses 11 litters with 16 hybrids F2 were obtained. A litter size was estimated as 1.27±0.47, and the mean litter size was lower comparing the control and parents (p<0.01).
In crossing type II (50 × 50), 35 litters and 106 hybrids F1 were obtained. Litter size was 2.94 ± 0.63. In inbred crosses, 44 litters with 71 hybrids F2 were obtained. A litter size was estimated as 1.61±0.84, with the mean litter size was lower when comparing the control and parents (p<0.01).
We analyzed a total of 106 spermatocytes of the hybrids (Table
Synaptic characteristics of multivalents and sex bivalents in the spermatocytes and oocytes of homozygotes and heterozygotes of E. tancrei.
Name | Number of cells | Sex (XX) bivalent associations (±SE) | SC multivalent (±SE) (Penta-/tetravalent) | ||
---|---|---|---|---|---|
With gaps | Closed | Open | |||
E. tancrei 2n=54, male | 102 | 0.17±0.03 1 | – | – | – |
E. tancrei hybrid 2n=49, male | 106 | 0.48±0.06 1,2 | 0.10±0.07 3 | 0.69±0.09 | 0.31±0.02 |
E. tancrei hybrid 2n=49, female | 59 | Sex bivalent behaves as autosomes. | 0.29±0.03 3,4 | 0.62±0.04 | 0.38±0.03 5 |
E. tancrei hybrid 2n=50, male | 94 | 0,08±0,04 2 | 0.11±0.04 4 | – | 0.96±0.06 5 |
The number of spermatocytes, for which the structure of the sex bivalent and SC multivalent were defined, was less than the total number of cells. This may occur due to weak or partial staining of the axial elements of SC and XX bivalents, which was previously observed in E. talpinus (
We studied 59 oocytes in total. According to our observations, in cell suspensions of ovaries, the number of oocytes was lower in hybrids (three to six nuclei per slide) than in ordinary of Ellobius talpinus Pallas, 1770 females (40–60 nuclei per slide) (
There are 21 autosomal SC bivalents in pachytene oocytes. One SC bivalent was formed by metacentric chromosomes, another SC bivalent appeared due to single pair of non-Rb submetacentrics, and the other 19 SC bivalents were formed by acrocentrics. The rest of chromosomes presented as SC pentavalent and sex (XX) bivalent (Fig.
Spreads of oocytes from intraspecific F1 hybrid E. tancrei (2n = 49, NF = 56). Immunostaining with antibodies to SCP 3 (green) and to the centromere (ACA, red). The yellow arrow marks the SC pentavalent a Early-mid pachytene oocyte: 21 SC bivalents, SC pentavalent, XX bivalent bSC pentavalent (yellow dotted line) from Fig.
The XX bivalent that was formed between the two largest acrocentrics was reliably identified via immunostaining of centromeric proteins. Sex chromosomes form the SC, which is indistinguishable from autosomal SCs.
The SC pentavalent was formed by three submetacentrics with MBH, as well as two acrocentrics (Fig.
We analyzed 94 spermatocytes of the F1 hybrid (Table
Chromosome synapsis in pachytene spermatocytes and diakinesis/metaphase I cell of F1 hybrid E. tancrei (2n = 50, NF = 56). a The scheme reflects a prognosis for chromosome synapsis in prophase I of meiosis [Trivalent №1 Rb(2/2.18/18), trivalent №2 Rb(4/4.12/12), and tetravalent Rb(5/5.9/9.13/13)]. M – metacentric, A – acrocentric b Electron micrograph of part of the spread spermatocyte. Nb – DAPI-positive nucleolus-like bodyc Spermatocyte is stained with DAPI (blue). Immunostaining with antibodies to SCP 3 (green) and to MLH1 (yellow) d Autosome’s axial element (A) stiking to Nb of sex bivalent (autosome – sex chromosome association) e Open SC trivalent f Diakinesis/Metaphase I cell showing 18 pairing elements, XX bivalent, two univalents (Un), two trivalents (Tr) and tetravalent. Scale bar: 5 μm.
The SC tetravalent was usually open (0.96±0.06). It was formed by two Rb metacentrics with MBH and two acrocentrics (Fig.
Each SC trivalent was formed by one Rb metacentric and two acrocentrics (Fig.
Small acrocentrics did not undergo synapsis in some pachytene nuclei, and univalents were discovered (Fig.
Previous studies support the suggestion that the single and multiple Rb fusions may be involved in the process of speciation because accumulation of Rb translocations (including MBH) in different populations of the species can lead to reproductive isolation between them (
The impact of simple and complex SC configurations on the progression of spermatogenesis and fertility varied among different groups of animals. Simple and complex SC configurations are well studied in different races of shrews (
In Mus musculus domesticus, the impact of Rb translocation in meiotic events and, in general, the formation of gametes, may be insignificant or significant, depending on the number of Rb fusions in the heterozygotes and, therefore, the characteristics and organization of SC chains in the meiotic nucleus. Heterozygous mice with four and seven trivalents had abnormal pairing such as SC trivalent associations with each other and with XY bivalent that might be the cause of infertility (
Fertility of lemurs was also variable depending on the number of Rb fusions. Hybrids with three to six SC trivalents were usually fertile. Heterozygous lemurs with eight trivalents in spermatocytes were viable, but had reduced fertility. Hybrids with complex SC configurations, including ones formed by Rb metacentrics that shared monobrachial homology, were sterile (
Studied heterozygous E. tancrei had approximately half the level of fertility in comparison with parental individuals and animals with 2n=54 (see also
Previously, we have studied other variants of intraspecific hybrids. In the nuclei of spermatocytes of F1 (2n = 44), hybrids from crossing 2n = 54 and 2n = 34 were expected to have six SC bivalents, the sex bivalent, and 10 SC trivalents, each of which was formed as a result of the synapsis of one Rb metacentric and two acrocentrics. However, the expected pattern could be observed only in a single nucleus at the late pachytene. It was found that chromosome synapsis in forming trivalents in most cases was slower than was bivalent synapsis (
Formation of complex SC configurations (SC chains and rings) in heterozygotes for multiple Rb translocation is accompanied by extended preservation zones of asynapsis in centromeric regions of acrocentrics and metacentrics (
Signs of transcriptional inactivation of asynapted chromatin involving the proteins gamma-H2AX, ATR, and SUMO1 were detected in mice heterozygous for eight Rb translocations. Such inactivation, however, did not lead to disturbances in the formation of the sex body in most of the spermatocytes nuclei (
This work was partially supported by research grants of the Russian Foundation for Basic Research № 15-29-02649, № 13-04-02071, № 14-04-00785, 15-29-02384 and by the Presidium of the Russian Academy of Sciences Program “Biodiversity of natural systems”, subprogram “Gene pool of wildlifeand conservation”. We thank Dr. A.S. Saidov, the Institute of Zoology and Parasitology of the Academy of Sciences of the Tadjik Republic for help in organizing the field research. We are grateful to G.N. Davidovich and A.G. Bogdanov of the Electron Microscopy Laboratory of Biological Faculty of Moscow State University for the technical assistance.