The behaviour of kinetochore microtubules during meiosis in the fungusSaprolegnia

Summary InSaprolegnia, kinetochore microtubules persist throughout the mitotic nuclear cycle but, whilst present at leptotene, they disappear coincidently with the formation of synaptonemal complexes at pachytene and reform at metaphase I. In some other fungi chromosomal segregation is random in meiosis and non-random in mitosis. The attachment of chromosomes to persistent kinetochore microtubules in mitosis, but not meiosis, inSaprolegnia provides a plausible explanation for such behaviour. At metaphase I each bivalent is connected to the spindle by 2 laterally paired kinetochore microtubules whereas at metaphase II (as in mitosis) each univalent bears only one kinetochore microtubule, thus showing that all kinetochores are fully active at all stages of meiosis.     

Morphology and behaviour of sex chromosomes during meiosis in Ascaris suum.

The morphology and behaviour of sex chromosomes was studied in A. suum during meiosis. It was found that the five sex chromosomes have their proper characteristic. The largest is submetacentric, of 2 microns mean length. The second largest is acrocentric, mean length of 1.4 mu. The third largest is metacentric, 1.2 mu mean length. The fourth and the fifth are metacentric, of mean length of 1 mu. In primary and secondary spermatocyte cells the sex chromosomes are close to each other, most often in the peripheral part of the cell. During anaphase I the pentad sex chromosomes lie freely between the two sister cells. It is assumed that in anaphase II the five sex chromosomes divide equally and are regularly distributed in the daughter cells. It was found that the chromosomes set of female Ascaris in metaphase I contains 24 bivalent chromosomes n = 24 and of male Ascaris 19 bivalents and 5 univalents. It is assumed that the univalent chromosomes, found in spermatocyte cells, determine sex.     

Postembryonic development and food consumption of Dichroplus elongatus Giglio-Tos and Dichroplus maculipennis (Blanchard) (Orhtoptera: Acrididae: Melanoplinae) under laboratory conditions

Dichroplus maculipennis (Blanchard) and D. elongatus Giglio-Tos are two of the most important melanoplines in Argentina, both ecologically and economically. The postembryonic development and forage loss (consumption of Bromus brevis Ness + fallen material) caused by older nymphs (instars IV, V, VI) and adults of both species were studied under controlled conditions (30oC, 14L:10D, 40% RH). Five nymphal instars were recorded in D. elongatus, and six in D. maculipennis. Total nymphal development was similar in both species (D. elongatus: 32 ± 0.70 days; D. maculipennis: 34.5 ± 0.37 days). Daily consumption increased from nymphal instars to pre-reproductive adult stage. In both species, pre-reproductive females had higher consumption rates than other stages considered (D. elongatus: 30.6 ± 0.56 mg dry weight/day; D. maculipennis: 48.7 ± 0.74 mg dry weight/day). In the reproductive stage, consumption decreased significantly in both sexes. When feeding, D. maculipennis let some plant material to drop, increasing total loss. The percentage of fallen material was greater in reproductive adults, representing 3.9% and 2.9% of the total daily loss for males and females, respectively. Females and males of D. maculipennis were heavier than those of D. elongatus (P < 0.05), and daily consumption was significantly higher (P < 0.05). Regardless sex and reproductive status, adults of D. maculipennis consumed 29.1 ± 0.64 mg dry weight/day on average, while one of D. elongatus 20.0 ± 0.3 mg dry weight/ day.     

C-heterochromatin pattern of ten species of tetrigids (Tetrigidae, Orthoptera)

The karyotypes of species belonging to the Tetrigidae is characterised by structural conservatism. The standard chromosome set of T. japonica, T. simulans, T. bolivari, P. meridionalis, U. depressus, and F. robustus consists of 2n = 13 acrocentric chromosomes in males and 2n = 14 in females, with a sex determining mechanism of X0 male and XX female. C-bands distribution often species belonging to 4 genera were studied. Differences in the position of C-bands and number of chiasma between species are discussed.     

Orientation and segregation of Robertsonian trivalents in Dichroplus pratensis (Acrididae)

Pairing behavior, metaphase I orientation, and anaphase I segregation of centric fusion trivalents were studied in 26 single, 15 double, and 2 triple male fusion heterozygotes of the polymorphic South American melanopline grasshopper Dichroplus pratensis. They represent the seven different fusions and their combinations already described in different populations of the species. Our analysis showed the following: (1) pairing behavior is very regular in all trivalents; (2) frequencies of linear orientation was very low irrespective of the trivalent involved; (3) reorientation seems to occur frequently since frequencies of abnormal segregation and aneuploid second division cells were invariably lower than those of nonconvergent orientation; (4) aneuploidy and abnormal sperm production increases with increasing number of fusions; (5) chiasma frequency and localisation is relevant to trivalent orientation since trivalents with nonconvergent orientations showed proximal and interstitial chiasmata more frequently than convergently oriented ones. The results are in agreement with the hypothesis that these polymorphisms are old and stable, and confirm that for the maintenance of a balanced polymorphism, if this polymorphism is adaptive because of its consequences on recombination, position effects, etc., changes tending to stabilise trivalent orientation and segregation are central.     

Nucleolus organizing regions and semi-persistent nucleolus during meiosis in Spartocera fusca (Thunberg) (Coreidae, Heteroptera)

The Coreidae are cytogenetically characterized by possessing holokinetic chromosomes and a pre-reductional type of meiosis. The modal diploid chromosome number of the family is 21, with a pair of m chromosomes and an XO/XX sex chromosome determining system. Spartocera fusca presents 2n=23/24=20+2m+XO/20+2m+XX (male/female). Meiosis follows the general pattern described for heteropterans, with a diffuse stage after pachytene and a particular chromosome arrangement at both metaphase plates. S. fusca presents some cytogenetic peculiarities: the X chromosome shows a secondary constriction in a medial position, which is not a nucleolus organizing region. It has been revealed by in situ hybridization with a rDNA probe that the NOR is localized at the telomeric region of one autosomal pair. Furthermore, during the meiosis of three specimens of S. fusca a semi-persistent nucleolus was detected from early meiotic prophase until telophase II; the presence of this semi-persistent nucleolus together with the long diffuse stage detected in the specimens suggest that a continuous biosynthetic activity is required for spermiogenesis. These observations could be related to differences in the environmental, and therefore, physiological conditions of the analyzed individuals.     

C-heterochromatin variation in the genus Eumigus (Orthoptera: Pamphagoidea)

Eumigus punctatus, E. monticulus and E. cucullatus all have 2n=19 and similar chromosome morphology, all the elements being telocentric. In E. cucullatus there are C-bands in all chromosomes, but not in E. monticulus. The possible origins of these differences and their cytotaxonomic significance are discussed.     

C-heterochromatin polymorphism and variation in chiasma localization in Euchorthippus pulvinatus gallicus (Acrididae,Orthoptera)

Eight populations of the grasshopper Euchorthippus pulvinatus gallicus have been analyzed by means of C-banding. Chromosome pairs M6, M7 and S8 show both quantitative and qualitative variation in their C-heterochromatin. There are at least four different types of M6, three of M7 and two of S8. Differences in the frequencies of these chromosome types have been found between populations. Within a given population the frequencies of the different M7 and S8 chromosomes fit a Hardy-Weinberg distribution and they remain constant within and between generations. The possible adaptative role of supernumerary heterochromatin as leading to a redistribution of chiasmata in the heterochromatin carrier chromosomes is discussed.     

Behavior of centrioles during meiosis in the male silkworm, Bombyx mori (Lepidoptera)

The behavior of centrioles during eupyrene and apyrene meiosis was examined in the silkworm, Bombyx mori , by transmission electron microscopy and indirect immunofluorescence for tubulin. In eupyrene spermatocytes the centrioles, accompanied by axonemes, attached temporarily to the nucleus at diplotene, then detached from the nucleus in diakinesis. After the separation, a beret-shaped structure consisting of a double membrane covered the proximal region of the pair of centrioles. The structure disappeared after breakdown of the nuclear membrane. The centriole, with the axoneme, reattached to the nucleus at telophase I. The process was repeated during meiosis II until the centrioles maintained their nuclear attachment in newly developed spermatids. In stark contrast to their eupyrene counterparts, apyrene spermatocytes were conspicuously devoid of any attachment of the centrioles to the nucleus. These eupyrene-specific and apyrene-specific relationships were consistently and repeatedly found between the nuclear membrane and centrioles, giving rise to suspicion that the behavioral phenomena may be related to differentiation of the dimorphic sperm types.     

Nucleolar behavior during meiosis in four species of phyllostomid bats (Chiroptera, Mammalia)

We analyzed the behavior of the nucleolus, nucleolar structures and nucleolus organizer regions (NORs) during meiotic division in four species of phyllostomid bats that have different numbers and locations of NORs. Nucleoli began disassembly at leptotene, and the subcomponents released from the nucleolus were dispersed in the nucleoplasm, associated with perichromosomal regions, or they remained associated with NORs throughout division. In Phyllostomus discolor, a delay in nucleolus disassembly was observed; it disassembled by the end of pachytene. The RNA complexes identified by acridine orange staining were observed dispersed in the nucleoplasm and associated with perichromosomal regions. FISH with rDNA probe revealed the number of NORs of the species: one NOR in Carollia perspicillata, one pair in Platyrrhinus lineatus and P. discolor, and three pairs in Artibeus lituratus. During pachytene, there was a temporary dissociation of the homologous NORs, which returned to pairing at diplotene. The variation in the number (from one to three pairs) and location of NORs (in sex or autosomal chromosomes, at terminal or interstitial regions) did not seem to interfere with the nucleolar behavior of the different species because no variation in nucleolar behavior that could be correlated with the variation in the number and chromosomal location of NORs was detected.     

The dynamic behaviour of mitochondria in living zygotes during maturation and meiosis in Chlamydomonas reinhardtii

To understand fully the function of mitochondria during the development of cells and organs, it is important to elucidate the dynamics of their morphology. However, the detailed morphology of mitochondria during meiosis has not yet been studied in algae. We examined the mitochondrial morphology of Chlamydomonas reinhardtii and classified zygotes into seven types by mitochondrial morphology in order to analyse the morphological change in mature and meiotic zygotes. We also investigated the oxygen consumption of living zygotes and the effects of tubulin and actin polymerization inhibitors on mitochondria, using fluorescence microscopy and oxygen electrodes. During zygote maturation, mitochondria fragmented into small particles, with a large decrease in oxygen consumption. When mature zygotes were exposed to light, mitochondria became tubular and formed a network, and oxygen consumption gradually recovered. At the same time, particle-like mitochondrial nucleoids became stringy and produced new nucleoid particles. Tubular mitochondria accumulated around the cell nucleus and then spread throughout the cell. Cell division followed (first and second rounds), and the resultant daughter cells had tubular mitochondria in a mesh-like arrangement. An inhibitor of tubulin polymerization, demecolcine, inhibited the assembly of mitochondria around the cell nucleus, whereas an inhibitor of actin polymerization, latrunculin B, inhibited the formation of tubular mitochondria. These results suggest that microtubules are probably involved in mitochondrial accumulation around the cell nucleus, whereas microfilaments may maintain the tubular network of mitochondria.     

Testicular mitosis, meiosis and apoptosis in mink (Mustela vison) during breeding and non-breeding seasons

Testes of mink were compared between the breeding (March) and non-breeding seasons with the start (November) and cessation (May) of spermatogenic activity. Testicular mass and spermatozoa per gram testis were assessed. Percentages of haploid (1C), diploid (2C) and tetraploid (4C) cells were monitored using DNA flow cytometry and the proportions of somatic and spermatogenetic cells were determined after selective labelling of somatic cells with a vimentin antibody. Apoptosis was examined by cell death detection ELISA, and testosterone concentrations were measured with an enzyme-immunoassay. The significantly higher testis mass during the breeding period coincided with higher numbers of testicular spermatozoa per gram testis and peak of testicular testosterone concentration in comparison with non-breeding periods. The proportions of 1C, 2C and 4C cells showed corresponding strong differences between these periods with the maximum of 1C cells during breeding. The proportions of testicular cells in G2-M phase of mitosis were very low during the period of peak spermatogenesis; they were markedly increased in the time of autumnal resumption in November but were even higher during testis involution in May. However. the meiotic transformation (1C:4C ratio) is maximal in March. The total as well as the relative proportions of spermatogenic and somatic cells differed significantly not only between breeding and non-breeding periods but also between the periods at the start and at the end of active spermatogenesis. The intensity of apoptosis was also seasonally dependent. The highest level in March indicates a stimulated apoptosis even during the breeding period. In conclusion, the production of spermatozoa in mink is intensified by enlargement of gonads as well as enhanced efficiency of spermatogenesis during breeding. In this time, the testosterone concentration and the meiotic transformation show high levels, but the mitotic activity of spermatogenic cells is already significantly diminished and an intensified apoptosis seems to precede the forthcoming testis involution after breeding. The results suggest that the regulation of seasonal testicular activity is characterised by co-ordinated shifts in the relationships between mitosis, meiosis, apoptosis and testosterone production.     

The effect of C-heterochromatin in chiasma terminalisation in Chorthippus biguttulus L. (Acrididae,Orthoptera)

Using the Giemsa C-banding procedure, a polymorphism in chromosome banding pattern has been found in a spanish population of Chorthippus biguttulus. The variation in C-banding pattern shown by bivalent M 6 allowed to study the effect of C-heterochromatin on chiasma terminalisation. The results indicate that interstitial heterochromatin acts as a barrier preventing chiasmata to pass. Anaphase separation seems to be normal but could be slightly delayed. A similar role for telomeric C-heterochromatin is suggested.     

Spindle membranes and calcium sequestration during meiosis ofDysdercus intermedius (Heteroptera)

The fate of intracellular membranes stained by the osmium ferricyanide (OsFeCN) procedure was followed from premeiotic interphase to interkinesis inDysdercus intermedius. During diakinesis the centrioles forming primary cilia attach temporarily with their proximal ends to the nuclear envelope which is stretched from pole to pole. Breakdown of the nuclear envelope is preceded by deep indentations with microtubules from growing asters. Vesicles of smooth endoplasmic reticulum which accumulate gradually in the course of prophase contribute to the ensheathment of the chromosomes with membranes. When the nuclear envelope breaks down, the polar parts of the formerly perinuclear membranes follow the ingrowth of the spindle microtubules towards the cell equator where the seven bivalents are arranged in a circle with the X₁X₂ sex chromosomes in the centre. The metaphase I spindle thus contains longitudinally oriented membranes between the poles, membranous envelopes around all chromosomes and radial connections from the autosomes to the sex chromosomes in the centre. At anaphase the homologues leave their common sheath and a microtubular stembody surrounded by membranes appears between the receding dyads. In the interkinetic nucleus the gonosomes are separated from the autosomes by a common membranous sheath which may be instrumental in their joint assignment to only one pole in the second meiotic division. Calcium sequestering sites visualized by oxalate precipitation are the Golgi lamellae and vesicles derived from them that surround the whole spindle body.     

Parallel polymorphisms for supernumerary heterochromatin in Dichroplus elongatus (Orthoptera):

Natural populations of Dichroplus elongatus often exhibit diverse forms of supernumerary heterochromatin. In Tafi Viejo, Tucumán (Argentina), parallel polymorphisms for extra segments on pairs M₆, S₉ (proximal) and S₁₀ (distal) and a mitotically unstable B chromosome were detected. The segments produce intrachromosome effects excluding chiasmata from their neigbourhood and displacing them to regions distal to the extra heterochromatin. The individuals bearing simultaneously the B and the segment on M₆ show a decrease in the frequency of interstitial chismata (X_i) while those carrying only segments on M₆ or S₁₀ have an increase of this variable when compared with standard males. The between cell variance of X_i is also higher in M₆ segment carriers. B individuals present an increase of abnormal spermatid frequency which may be explained by the simultaneous occurrence of two mechanisms: 1) lagging of supernumeraries and 2) physiological effects of B's affecting the meiosis even in cells lacking them. The intra and interchromosome effects may be important for the plasticity of populations allowing recombination to explore new chromosome regions. The B polymorphism seems to be stable which points to an advantage of B carriers and/or the occurrence of accumulation mechanisms counterbalancing their negative effects on fertility.