Surface-spread Synaptonemal Complexes Research Articles

Heteromorphism of “Homomorphic” Sex Chromosomes in Two Anole Species (Squamata, Dactyloidae) Revealed by Synaptonemal Complex Analysis

Iguanians (Pleurodonta) are one of the reptile lineages that, like birds and mammals, have sex chromosomes of ancient origin. In most iguanians these are microchromosomes, making a distinction between the X and Y as well as between homeologous sex chromosomes in other species difficult. Meiotic chromosome analysis may be used to elucidate their differentiation, because meiotic prophase chromosomes are longer and less condensed than metaphase chromosomes, and the homologues are paired with each other, revealing minor heteromorphisms. Using electron and fluorescent microscopy of surface spread synaptonemal complexes (SCs) and immunolocalization of the proteins of the SC (SYCP3), the centromere, and recombination nodules (MLH1), we examined sex chromosome synapsis and recombination in 2 species of anoles (Dactyloidae), Anolis carolinensis and Deiroptyx coelestinus, in which the sex chromosomes represent the ancestral condition of iguanians. We detected clear differences in size between the anole X and Y microchromosomes and found an interspecies difference in the localization of the pseudoautosomal region. Our results show that the apparent homomorphy of certain reptile sex chromosome systems can hide a cryptic differentiation, which potentially may influence the evolution of sexual dimorphism and speciation.

Genetic and cytological evidence of hybridization between native Ruditapes decussatus and introduced Ruditapes philippinarum (Mollusca, Bivalvia, Veneridae) in NW Spain

The introduction of foreign species in marine ecosystems is an established method to increase productivity and generate income in aquaculture. A good example is the introduction of the Japanese carpet shell, Ruditapes philippinarum, now the major contributor to clam landings in Europe. The similarity of R. philippinarum and the native grooved carpet shell, R. decussatus, the plasticity of their shell morphology and the presence of both species on the same beds make it very difficult sometimes for the morphological identification of specimens that show intermediate shell characteristics. A species identification method using length differences in the internal transcribed spacer (ITS) region of the major ribosomal RNA genes and the whole repeat sequence of the minor ribosomal RNA genes (5S rDNA) was developed for Ruditapes. Following morphological recognition, this method enabled the identification of both species of Ruditapes. However, nine of the 222 individuals morphologically identified as R. decussatus had sequences specific to both R. decussatus and R. philippinarum in both the ITS region and the 5S rDNA and were putative hybrids. Species-specific ITS and 5S rDNAs were used as probes in fluorescent in situ hybridization (FISH) experiments to map these sequences on surface spread synaptonemal complexes (SCs) of R. decussatus and R. philippinarum. Differences on the chromosomal location of those sequences between species were also detected. FISH on the SCs of two of the nine hybrid individuals also showed signal patterns corresponding to those detected in both R. decussatus and R. philippinarum thus confirming their hybrid nature. The hybrids showed 16S mitochondrial rDNA sequences of R. decussatus. In conclusion, genetic and cytological evidence of the occurrence of hybridization between female R. decussatus and male R. philippinarum was obtained.

Reproductive isolation due to the genetic incompatibilities betweenThrichomys pachyurusand two subspecies ofThrichomys apereoides(Rodentia, Echimyidae)

We tested intrinsic reproductive isolation between 3 taxa of the South American caviomorph rodent Thrichomys (Rodentia, Echimyidae): T. pachyurus, T. apereoides subsp. apereoides and T. apereoides subsp. laurentius. They were mated in captivity and produced viable progeny. Some F1 hybrid females were fertile, whereas all F1 males were sterile. Histological examination revealed meiotic arrest at the primary spermatocyte stage. No sperm was detected in testes or epididymes. Electron microscopic analysis of surface spread synaptonemal complexes revealed a complete failure of chromosome pairing in F1 hybrids of T. pachyurus with T. apereoides subsp. laurentius and T. apereoides subsp. apereoides. In the male hybrids between T. apereoides subsp. apereoides and T. apereoides subsp. laurentius, meiosis did not proceed beyond diplotene, although all of the chromosomes, including heteromorphic ones, paired in an orderly fashion. Backcross males with homomorphic karyotypes showed segregation in meiosis progression. This indicates that male hybrid sterility is due to genetic, but not chromosomal, incompatibility of the parental taxa.

The dicentric nature of the metacentric B chromosome of Metaleptea brevicornis adspersa (Acridinae, acrididae).

The meiotic behaviour of a large B isochromosome of Metaleptea brevicornis adspersa (Acridinae, Acrididae) was studied in both sexes using conventional preparations and in males by silver staining of surface-spread synaptonemal complexes and kinetochores. In males, both arms of the B chromosome synapse at zygotene-early pachytene suggesting its isochromosomal nature. Both arms separate precociously at late pachytene and remain joined only through the formation of a single distal chiasma in 92.5% and 91.7% of male and female meiocytes, respectively. In both sexes, the B chromosome tends to migrate precociously to one of the spindle poles, auto-orientating at the equator in 19.2% and 16.6% of metaphase I cells of males and females, respectively. In males, this produces a high frequency of anaphase I/telophase I lagging with the consequent inhibition of cytokinesis and the formation of second division restitution nuclei. For a better understanding of the synaptic and segregation behaviour of the B chromosome in males, pachytene synaptonemal complex (SC) formation was studied. In 0B males, pachytene nuclei invariably showed 11 fully paired SCs, each one with a densely stained pair of terminal kinetochores, and a single axis with a single kinetochore corresponding to the X univalent. In 1B males, both arms of the B chromosome appeared fully paired in 93. 8% of the nuclei, showing complete asynapsis in the rest. This frequency is almost exactly the same as distal chiasma formation in diplotene. The centric region of the SC of the B chromosome had two kinetochores connected by a single axial filament of variable length. The dicentric nature of the B was confirmed by silver staining of kinetochores at metaphase I and anaphase II stages. This is the first time that a dicentric B isochromosome has been described. Based on these results, two alternative models for the origin of isochromosomes are postulated.

Cytogenetics of collared lemmings (Dicrostonyx groenlandicus). I. Meiotic behavior and evolution of the neo-XY sex-chromosome system.

Electron-microscopic analysis of surface-spread synaptonemal complexes at pachynema and light-microscopic analysis of chromosomal configurations at diakinesis/metaphase I corroborate the hypothesized neo-XY derivation of the sex chromosomes of Dicrostonyx groenlandicus. Although an intact neo-XY pairing configuration was observed in a relatively small percentage of the pachytene cells in each individual, the high incidence of neo-XY bivalents at diakinesis/metaphase I suggests that the other observed pachytene configurations were artifacts of the physical stresses of the surface-spreading procedure. The very low frequency (0.6%) of univalent neo-X and neo-Y chromosomes at diakinesis and metaphase I is attributable to consistent synapsis and recombination between their homologous autosomally derived segments. The resultant stability of the sex bivalent through metaphase I may have increased the efficacy of sex-chromosome segregation, and thereby played a mechanistic role in the evolutionary incorporation of the neo-XY sex-chromosome constitution in D. groenlandicus.

HSP70-2 is part of the synaptonemal complex in mouse and hamster spermatocytes

Mouse spermatogenic cells are known to express HSP70-2, a member of the HSP70 family of heatshock proteins. The purpose of the present study was to characterize further the expression and localization of HSP70-2 in meiotic cells of mice and hamsters. After separating mouse spermatogenic cells into cytoplasmic and nuclear fractions, proteins were separated by two-dimensional gel electrophoresis and detected with HSP-specific antibodies. Of several HSP70 proteins identified in the cytoplasm, only HSC70 and HSP70-2 were also detected in the nucleus. Immunocytological analyses of spermatocyte prophase cells revealed that HSP70-2 was associated with the synaptonemal complex. Surfacespread synaptonemal complexes at pachytene and diplotene stages labeled distinctly with the antiserum to HSP70-2. Synaptonemal complexes from fetal mouse oocytes failed to show any evidence of HSP70-2. Reverse-transcriptase-polymerase chain reaction (RT-PCR) analyses of gene expression confirmed this sex specificity; Hsp70-2 mRNA was detected in mouse testes, but not ovaries. These findings are suggestive of a previously unsuspected sexual dimorphism in structure and/or function of the synaptonemal complex.

HSP70-2 is part of the synaptonemal complex in mouse and hamster spermatocytes

Mouse spermatogenic cells are known to express HSP70-2, a member of the HSP70 family of heat-shock proteins. The purpose of the present study was to characterize further the expression and localization of HSP70-2 in meiotic cells of mice and hamsters. After separating mouse spermatogenic cells into cytoplasmic and nuclear fractions, proteins were separated by two-dimensional gel electrophoresis and detected with HSP-specific antibodies. Of several HSP70 proteins identified in the cytoplasm, only HSC70 and HSP70-2 were also detected in the nucleus. Immunocytological analyses of spermatocyte prophase cells revealed that HSP70-2 was associated with the synaptonemal complex. Surface-spread synaptonemal complexes at pachytene and diplotene stages labeled distinctly with the antiserum to HSP70-2. Synaptonemal complexes from fetal mouse oocytes failed to show any evidence of HSP70-2. Reverse-transcriptase-polymerase chain reaction (RT-PCR) analyses of gene expression confirmed this sex specificity; Hsp70-2 mRNA was detected in mouse testes, but not ovaries. These findings are suggestive of a previously unsuspected sexual dimorphism in structure and/or function of the synaptonemal complex.

Chiasma localization, heterochromatin and synaptonemal complexes in the grasshopper Pyrgomorpha conica.

Surface-spread synaptonemal complexes and chiasma distributions in spermatocytes with different C-banding patterns and chiasma distributions in oocytes were analyzed in the grasshopper Pyrgomorpha conica. Male meiosis was characterized by a proximal/distal chiasma localization and complete pairing of homologous chromosomes at pachytene. However, there were indications of a relationship between the frequency and location of pairing initiation sites and chiasma distribution. The presence of a proximal supernumerary segment in a medium-sized chromosome does not increase the mean cell chiasma frequency of carrier individuals compared with those lacking it but may modify chiasma distribution in at least some carrier bivalents. This effect could be related to heterosynapsis in the region near the segment. Mean cell chiasma frequency was significantly lower in females than in males. Females also showed altered chiasma distributions compared with males, with fewer proximal chiasmata and more interstitial and distal chiasmata.

Synapsis in grasshopper bivalents heterozygous for centric shifts

Analysis of surface-spread synaptonemal complexes of zygotene and pachytene spermatocytes was carried out on centric-shift heterozygotes of grasshoppers. These rearrangements affected the M7 chromosome in Chorthippus vagans and the M6 and S8 chromosomes in Chorthippus apricarius. The shifts in the latter two chromosomes were also associated with C-heterochromatin variations between homologous chromosomes. Rearranged chromosomes proceeded directly to heterosynapsis without an apparent intervening homosynaptic phase in M7 bivalents of Ch. vagans and M6 bivalents of Ch. apricarius. In the latter case, axial equalization of the heterochromatin polymorphism was also achieved. On the other hand, asynapsis of the intercentromeric regions throughout pachytene was the rule in the centric shift involving the S8 chromosome of Ch. apricarius. In the three cases analysed, the production of unbalanced gametes in the heterozygotes is precluded either by the lack of chiasma formation in heterosynapsed rearranged segments or by the lack of pairing between such segments. Chiasmata were limited to the homologous regions of the heteromorphic bivalents.

Meiotic mutants of rye Secale cereale L. II. The nonhomologous synapsis in desynaptic mutants sy7 and sy10

We studied the expression and inheritance of two spontaneous mutations found in different populations of rye Secale cereale L. that cause high univalent frequency in meiosis and low fertility. Both mutations were inherited as monogenic recessives. For each of the mutations the corresponding gene symbols (sy7 and sy10) were suggested although their allelism has not been studied. These mutants differ in chiasma frequency and in the number of univalents per meiocyte. Electron microscopy of the wholemount surface-spread synaptonemal complexes (SCs) from microsporocytes of both mutants revealed that during meiotic prophase I random synapsis began and progressed that involved not only homologous but also nonhomologous chromosomes. SCs were formed with frequent changes of pairing partners (switches) and intrachromosomal foldbacks of unpaired axial elements. As a result, incompletely synapsed, non-homologous and multivalent SCs were formed in mutants by the stage analogous to pachytene in normal plants. In sy7 a maximum in the number of switches and foldbacks were observed at zygotene, whereas in sy10 this occurred at pachytene. We suggest that it is the process of recognition of homology that is impaired in both mutants. This leads to indiscriminate synapsis and prevents chiasma formation. Both mutants may be classified as desynaptic.

Synapsis in a paracentric inversion heterozygote of Chorthippus jacobsi (grasshopper)

Analysis of surface-spread synaptonemal complexes of zygotene and pachytene spermatocytes was carried out on a grasshopper heterozygous for a paracentric inversion in chromosome L3. Inverted segments usually heterosynapse at pachytene without previous homosynapsis, although the possibility of synaptic adjustment in some nuclei is not excluded. A relationship between this synaptic behaviour and the absence of chromosome bridges and acentric fragments in the nine anaphase I cells analysed is established. The lack of reports concerning the existence of paracentric inversions in grasshopper natural populations is also discussed.

EM investigations of surface spread synaptonemal complexes in a human male carrier of a pericentric inversion inv(13)(p12q14): the role of heterosynapsis for spermatocyte survival.

EM investigations of surface spread synaptonemal complexes of pachytene spermatocytes were performed on a human male carrier of a pericentric inv(13), ascertained through his daughter with a duplication-deficiency of the same chromosome. The inv(13) bivalent could be unambiguously identified by either its asymmetrical kinetochores or its nucleolar association or both. There was scarcity of reversed homosynapsis of the inverted segments with inversion loops, possibly related to the small size of the inversion. The majority of pachytene spermatocytes showed the inverted segment to be either asynapsed or heterosynapsed. Generally initial homosynapsis is highly efficient with high fidelity, while heterosynapsis at the secondary pachytene pairing phase may take place with any available partner remaining unsynapsed and including the differential segments of the XY. It is suggested that survival of spermatocytes to later stages of meiosis may be ensured by heterosynapsis, and the recombinant chromosome in the offspring might be the result of a U-type exchange.

Synaptonemal complex spreading in plants: Technical aspects and preliminary observations on the synaptonemal complex inPaeonia andOrnithogalum

A modified method for obtaining surface spread synaptonemal complexes (SCs) from pollen mother cells has been developed. Silver-stained SC-preparation of one monocotyledonous species,Ornithogalum flavovirens, and one dicotyledonous plant,Paeonia tenuifolia, were analysed by light and electron microscopy. The SCs in both species frequently broke into roughly equally sized SC pieces with staggered or blunt breakpoints. The telomeric ends of the SCs normally were lacking attachment organelles and, therefore, were hardly distinguishable from blunt breakpoints. Interstitially, shorter stretches of SCs often exhibited unpaired lateral elements. This phenomenon is discussed with regard to segmental incomplete homology and as it relates to the normal sequence of SC morphological changes during the course of meiotic prophase.