Achiasmatic Meiosis Research Articles

Nuclear phenomena during conjugation of the suctorian Heliophrya erhardi: I. EM-Observations of micronuclear meiosis

Summary We investigated micronuclear meiosis in the conjugating suctorian Heliophrya erhardi by time lapse cinemicrography of living cells, by indirect immunofluorescence (IIF) with monoclonal antibodies against modified tubulins, and by transmission electron microscopy, respectively high voltage electron microscopy (HVEM) of ultrathin and semithin sections. The fine structure of mid meiotic prophase reveals moderately condensed chromatin threads forming a “parachute” arrangement. Since only extremely short and also somewhat atypically structured synaptonemal complexes (SCs) are first discernible as early as diplotene, when partial chromosomal disjunction is seen, it is suggested that we are dealing here with achiasmatic meiosis. The spindle axis is established during prometaphase I by parallel alignment of polar MTs (pMTs) and the congression of chromosomes towards the equator is brought about by outgrowing kinetochore MTs (kMTs) becoming increasingly parallel. Metaphase I is indicated by a complete depolymerization of pMTs, revealing a truncated spindle type consisting exclusively of kMTs which never reach the presumptive poles before the metaphase/anaphase transition, and, as also seen by IIF, several of the small chromosomes converge with their kMTs forming numerous “subspindles”. These are focused on electron dense bodies located at half the distance to the presumptive poles. At late metaphase stages spindles become stabilized by their anchorage at the lamin-like sheets of the inner membrane of the nuclear envelope. Anaphase I is clearly subdivided into anaphase A and into anaphase B. While chromosome segregation in anaphase A I is not conspicuous an enormous interzonal spindle elongation that contributes substantially to chromosomal segregation is formed at anaphase B I. The mean velocity of pole separation of 0.5 μm/min corresponds rather well with the anaphase AI poleward migration. The telophase spindle develops from a centrally located torsional zone of the stembody in anaphase B by separation into two twisted areas which push the chromosomal fronts into opposite directions. During the dumbbell-shaped telophase stage, the daughter nuclei are pinched off by rotational movements of the terminalized contorted regions of the spindle. The nuclear events leading to meiosis II are highly similar to meiosis I.

Synaptonemal complexes in insects

The synaptonemal complex (SC) is the key nuclear element formed in meiotic prophase I to join 2 homologous chromosomes at the pachytene bivalent. It is a highly conserved structure that is universally present in eukaryotes. The SC is presented as a tripartite protein structure, which consists of 2 lateral elements and a central region. In insects, the central region is particularly distinct and highly ordered. This made it possible to describe the fine structure of the central region and propose a model of its architecture. Chromatid DNA is arranged in chromatin loops extending radially from the SC. The loops appear to consist of a basic chromatin fiber with a diameter of 20–30 nm. In many insect species, synaptonemal polycomplexes occur in postpachytene cells. They represent one of the possible ways of SC degradation. Another process, which occurs beyond pachytene, is the formation of proteinaceous chromatid axis, the silver-stained chromatid core. Based on results in insect models, the chromatid cores have been related to the structure and formation of the SC. Research on insect models significantly contributed to understanding individual steps of the SC formation and temporal sequence of chromosome pairing. These include the formation of lateral elements of the SC, pairing initiation, interlocking of chromosomes, and synapsis of homologous chromosomes. Attention is also given to non-homologous pairing, including synaptic adjustment, correction of pairing, and pairing of sex chromosomes. In the next section, chiasmatic and achiasmatic modes of meiosis are compared with respect to the SC formation. In the chiasmatic mode, the SCs display recombination nodules that are believed to mediate the process of recombination. These nodules were discovered in insects, and indirect evidence for their role comes from insects. Two different examples of achiasmatic meiosis, occurring in the heterogametic sex of several insect orders, are given: one involves the SC formation, whereas in the other, SCs are absent. Finally, the potential of SC karyotyping for analysis of the insect genome is discussed.

A genetic linkage map for the domesticated silkworm,Bombyx mori, based on restriction fragment length polymorphisms

SummaryWe present data for the initial construction of a molecular linkage map for the domesticated silkworm,Bombyx mori, based on 52 progeny from an F2 cross from a pair mating of inbred strains p50 and C108, using restriction fragment length polymorphisms (RFLPs). The map contains 15 characterized single copy sequences, 36 anonymous sequences derived from a follicular cDNA library, and 10 loci corresponding to a low copy number retrotransposon, mag. The 15 linkage groups and 8 ungrouped loci account for 23 of the 28 chromosomes and span a total recombination length of 413 cM; 10 linkage groups were correlated with established classic genetic maps. Scoring data from Southern blots were analysed using two Pascal programs written specifically to analyse linkage data in Lepidoptera, where females are the heterogametic sex and have achiasmatic meiosis (no crossing-over). These first examine evidence for linkage by calculating the maximum lod score under the hypothesis that the two loci are linked over the likelihood under the hypothesis that the two loci assort independently, and then determine multilocus linkage maps for groups of putatively syntenic loci by calculating the maximum likelihood estimate of the recombination fractions and the log likelihood using the EM algorithm for a specified order of loci along the chromosome. In addition, the possibility of spurious linkage was exhaustively tested by searching for genotypes forbidden by the absence of crossing-over in one sex.

Michael James Denham White, 20 August 1910 - 16 December 1983

Michael White put chromosomes into evolutionary thinking and made a primary contribution to the emerging neo-Darwinian evolutionary synthesis. He emphasized cytogenetic systems and argued that genic and chromosomal evolution were of seminal importance in the process of speciation and evolution. His major scientific contribution was Animal Cytology and Evolution (1945), a book that summarized, analysed and synthesized current information on animal chromosomes. White held a somewhat parallel place in cytogenetics to C.D. Darlington whose book Recent Advances in Cytology (1932, 1937) had earlier synthesized observations on plant chromosomes. For many years from the late 1930s there was a lively competition between these two industrious, innovative and self-assertive figures. Undoubtedly, White found satisfaction greater than that of science alone when ( contra Darlington) he described an achiasmate meiosis in a mantid during his first period of research in the United States in the late 1930s.

Sex differences in recombination

AbstractOne of the stronger empirical generalizations to emerge from the study of genetic systems is that achiasmate meiosis, which has evolved 25–30 times, is always restricted to the heterogametic sex in dioecious species, usually the male. Here we collate data on quantitative sex differences in chiasma frequency from 54 species (4 hermaphroditic flatworms, 18 dioecious insects and vertebrates and 32 hermaphroditic plants) to test whether similar trends hold. Though significant sex differences have been observed within many species, only the Liliaceae show a significant sexual dimorphism in chiasma frequency across species, with more crossing over in embryo mother cells than in pollen mother cells; chiasma frequencies are unrelated to sex and gamety in all other higher taxa studied. Further, the magnitude of sexual dimorphism, independent of sign, does not differ among the three main ecological groups (dioecious animals, plants, and hermaphroditic animals), contrary to what would be expected if it reflected sex‐specific selection on recombination. These results indicate that the strong trends for achiasmate meiosis do not apply to quantitative sex differences in recombination, and contradict theories of sex‐specific costs and benefits. An alternative hypothesis suggests that sex differences may be more‐or‐less neutral, selection determining only the mean rate of recombination. While male and female chiasma frequencies are more similar than would be expected under complete neutrality, a less absolute form of the hypothesis is more difficult to falsify. In female mice the sex bivalent has more chiasmata for its length than the autosomes, perhaps compensating for the absence of recombination in males. Finally, we observe that chiasma frequencies in males and females are positively correlated across species, validating the use of only one sex in comparative studies of recombination.

Synaptonemal complex formation in male scorpions exhibiting achiasmate meiosis and structural heterozygosity

Synaptonemal complex (SC) formation was studied in testicular material from individuals of a number of species from two families of Australian scorpions; Buthidae and Scorpionidae. These scorpions exhibit unusual cytogenetic features including achiasmate male meiosis, interchange heterozygosity, and centromeric fusion–fission and inversion heterozygosity. The synaptic behaviour of chromosomes involved in these rearrangements was studied from zygotene to metaphase I, using both meiotic chromosome preparations and techniques for examination of the SCs. Multivalent associations present during the achiasmate meiosis of both buthid and scorpionid scorpions are retained from prophase to metaphase I, unlike those present in polyploid achiasmate Bombyx females. Further evidence suggests that synaptic adjustment does not occur generally in achiasmate scorpionid inversion heterozygotes. However, for some inversions, pairing is seen to become more heterosynaptic from late prophase to metaphase I and this may be related to the pairing maintenance system during achiasmate meiosis in these specialized organisms.Key words: synaptonemal complex, achiasmate meiosis, heterozygosity, interchange, inversion.

Karyotype and achiasmatic female meiosis in Helicoverpa armigera (Hübner) and H. punctigera (Wallengren) (Lepidoptera: Noctuidae)

The economically important pest species Helicoverpa armigera and H. punctigera have a karyotype consisting of 31 pairs of chromosomes. The chromosomes are in a graded series of sizes such that pairs cannot be differentiated. Cytological evidence suggests that female meiosis is achiasmatic. Precocious separation of bivalents into univalents at metaphase I was observed in some spermatocytes of H. punctigera. This species also had a consistently greater number of bivalents with fully terminalised chiasmata in each spermatocyte at male metaphase I than H. armigera.Key words: Lepidoptera, Helicoverpa, achiasmatic meiosis, karyotype.

Cytogenetics of Australian scorpions. II. Chromosome polymorphism in species of Urodacus (family Scorpionidae)

The cytogenetic features of six species of scorpion from the Australian genus Urodacus (family Scorpionidae) were examined. Australian scorpionids possess monocentric chromosomes and male meiosis is achiasmate. Chromosome numbers are generally high and in some species extremely variable. Chromosome variation in one species, U. manicatus, was found to be due to fusion – fission polymorphism and variation in the number of small telocentric chromosome pairs. Additionally two populations exhibited extensive inversion heterozygosity. Available evidence suggests that female meiosis may also be achiasmate. The extensive numerical and structural heterozygosity found in U. manicatus may be related to the achiasmate meiosis, which allows regular pairing and disjunction of the multiple chromosome associations.Key words: achiasmate meiosis, chromosome polymorphism, structural heterozygosity, Robertsonian fusions.

Cytogenetics of Australian scorpions. I. Interchange polymorphism in the family Buthidae

Male scorpions from Australian species of the family Buthidae exhibit a unique combination of cytogenetic features including achiasmate meiosis, holocentric chromosomes, and extensive interchange heterozygosity. Chromosome number is highly conserved, with all species having a basic diploid number of 2n = 14. There is evidence that inbreeding has contributed to the establishment of populations with interchange heterozygotes, some exhibiting rings of up to 12 chromosomes. Although most populations contain both structural homozygotes and interchange heterozygotes, one population may exhibit fixed heterozygosity. It is argued that the interchange heterozygosity observed in buthids is of adaptive significance.Key words: interchange, chromosome polymorphism, achiasmate meiosis, holocentric chromosomes.

Crossing Over in Males of Higher Diptera (Brachycera).

Achiasmatic meiosis, or meiosis without crossing over, is characteristic of all higher Diptera males (suborder Brachycera). However, several cases of spontaneous crossing over in males have been reported in several different species. An examination of the published data suggests that recombinant chromosomes recovered from heterozygous males are usually the result of spontaneous crossing over in premeiotic cells. Mitotic, or somatic, crossing over probably occurs at a low frequency in all Diptera. When the crossover occurs in a gonial cell line, the recombinant chromosomes can be recovered in the gametes as presumptive meiotic crossovers. In cases where there is a translocation between the Y chromosome and an autosome, the segregation of the chromosomes from the translocation complex can produce aneuploids that phenotypically appear to be crossovers. Chromosome rearrangements and insertion elements, including the male sex-determining factor in Musca domestica and Megaselia scalaris, increase the frequency of exchange. Drosophlia ananassae appears to be an exception to the above. Genetic evidence in D. ananassae suggests that crossing over is a meiotic event and is controlled by a series of suppressors and enhancers.

A cytological study of species in the Eusimulium aureum group (Diptera: Simuliidae)

Earlier studies by R. W. Dunbar of the monophyletic (n = 2) Eusimulium aureum group were extended. Additional samples of his forms "A" and "B" (temperate North America), "C" and "D" (northern North America), "E" and "F" (Britain, northern Europe, Leningrad), and "G" (California to Guatemala) were analyzed. Two of Dunbar's forms, "I" (Gibraltar, Britain) and "J" (Mediterranean, Madeira, Azores, Leningrad), have been redefined. "K" (Gibraltar) and "L" and "M" (Canary Islands) are new forms. These are described in terms of fixed inversion differences, sex differential segments, and chromosomal polymorphisms. In a cytophylogeny, the Old World "J" forms the link to the New World "C." A preliminary polytene banding comparison of E. aureum and its putative n = 3 relatives reveals that the Old World forms are ancestral to those of the New World. All palaearctic forms have chiasmate meiosis in males, while all nearctic ones, with the remarkable exception of "G," are male achiasmate. Since male achiasmate meiosis is presumably a prerequisite for the reduction in chromosome number, the male-chiasmate condition of Old World forms is assumed to be due to secondary reversion, a postulate that is supported by the derivative male-chiasmate condition of "G."

Effect of the antitubulin drug nocodazole on meiosis and post-meiotic development in Tetrahymena thermophila, : Induction of achiasmatic meiosis

Nocodazole (ND), a potent antitubulin drug, can be used to dissect the steps of meiosis in Tetrahymena, presumably by interfering with the assembly of microtubules. Its effects depend upon the time during conjugation at which the drug is applied. When applied prior to the elongation of the micronucleus into the characteristic ‘crescent’ configuration, no crescent is formed and the chromosomes of prepachytene and pachytene condense into spherical nuclei. If ND is applied after micronuclear elongation has begun, but before it is fully elongated, the chromosomes fail to synapse and appear in metaphase I as unpaired monovalents. In contrast, the metaphase I chromosomes appear as bivalents when ND is applied later, during or after the crescent has reached its maximum elongation. Still later, application of ND inhibits chromosome movements during anaphase and telophase of either meiotic division, but does not prevent separation of kinetochores. In some of the blocked restitutive nuclei an additional round of chromosome replication occurs, corresponding to the third pregamic division in normal conjugation. The hyperploid micronuclei produced by such treatment may be useful in certain genetic manipulations and in studying the regulation of nuclear DNA content.

Karyological Studies on Trechinae (Coleoptera, Carabidae). III. Karyotype ofTrechos LatusPutz. andTrechus Quadristriatus(Schr.)

SUMMARYIn the present paper the chromosomes of two species of the genus Trechus are described and illustrated. The investigated species have a similar karyotype with a high proportion of metacentric chromosomes which can be expressed as the formula: male 22A + XO; female 22A + XX. In males of both species achiasmatic meiosis was recorded.

Achiasmate meiosis in the Fritillaria japonica group. II. Formation of synaptinemal complexes in microsporocytes.

An achiasmatic meiosis is observed in microsporocytes of Fritillaria japonica group, in which paired chromosomes devoid of chiasmata are continuosly maintained through meiotic prophase. Experiments were aimed at examining whether the chromosome pairing is associated with normal synaptinemalcomplex (SC) structure. Examination in the electron micrscope of pachytene cells showed that the SC was present in all cross-sections through the chromosomes, and the SC seemed to be normal in appearance and dimension.

Synaptonemal complexes, telomeric nucleoli and the karyosphere in achiasmatic oocyte meiosis of the carob moth

Synaptonemal complexes and telomeric nucleoli are involved in the spatial organization and regular distribution of homologous chromosomes in meiosis of the achiasmatic female carob moth. The bivalents are held together from zygotene to metaphase by the Synaptonemal complexes. These are attached to telomeric nucleoli which appear during early meiotic prophase and are unique to the oocyte. The telomeric nucleoli fuse during prophase and the chromosomes concentrate into a small karyosphere before prometaphase. During the final stages of prophase elements of the Synaptonemal complex are found in the periphery of the fibrillar region of the telomeric nucleoli.

Linkage relationships among five enzyme-coding gene loci in the copepod Tigriopus californicus: a genetic confirmation of achiasmiatic meiosis.

Linkage relationships among five polymorphic enzyme-coding gene loci in the marine copepod Tigriopus californicus have been determined using electrophoretic analysis of progeny from laboratory matings. Phosphoglucose isomerase (PGI; EC 5.3.1.9) was found to be tightly linked to glutamate-pyruvate transaminase (GPT; EC 2.6..1.2), with only one recombinant observed in 364 progeny; glutamate-oxaloacetate transaminase (GOT; EC 2.6.1.1) is linked to the PGI-GPT pair, with a recombination fraction of approximately 0.20 in male double heterozygotes. Phosphoglucomutase (PGM; EC 2.7.5.1) and an esterase (EST; EC 3.1.1.1) are not linked to the PGI, GPT, GOT grouping, which has been designated linkage group I. Reciprocal crosses have revealed that no recombination occurs in female T. californicus; this observation confirms a previous report that meiosis in female Tigriopus is achiasmatic.

Male achiasmatic meiosis in Caraboidea (Coleoptera, Adephaga)

The meiotic process was studied in fifty-three species belonging to the caraboid families Bembidiidae (48). Trechidae (3), Pogonidae (1), and Harpalidae (1). Males show achiasmatic meiosis patterns of the Callimantis type. In females only early prophase stages were observed, that are similar to those of the males. The characteristics of this abnormal meiosis and its evolutionary and cytotaxonomic significance are discussed.

Achiasmatic meiosis and complex heterozygosity in female cyclopoid copepods (Copepoda, Crustacea)

In Mesocyclops edax S.A. Forbes, 2n=14, a North American copepod, the females are heterozygous for several interchanges leading to the formation of large rings of chromosomes (rings of 14, or of 12 plus 1 bivalent) at meiotic metaphase, comparable to those of the plant Oenothera, although no chiasmata are present. The chromosomes are more or less metacentric and have large terminal H-segments. In the rings homologous arms are held together by connecting fibers which insert close to the euchromatin-heterochromatin junctions. Coordinated orientation of the zigzag type seems to be the role.

"Nodules" in the achiasmatic meiosis of Bithynia (Mollusca, Prosobranchia).

In spermatocytes of Bithynia leachi and B. tentaculata, synaptonemal complexes with nodules in an achiasmatic meiosis were demonstrated for the first time by means of the spreading technique. The number and position of the nodules are different in each species and are independent of the length of the chromosomes. Although no crossing over occurs in Bithynia chiasma-like terminal connections of the chromosomes are preserved in diakinesis by the presence of the nodules. --Enzymatic treatments revealed information regarding their composition of alkaline proteins, RNA and DNA. In all proteolytic digestions the nodules more of less retain their position in the central region, while they disappear immediately if SCs are exposed to DNase. On the basis of the results a model of the composition and the function of nodules can be developed considering the concept of a lampbrush-organization of the chromosomes in early meiotic prophase.

Achiasmate meiosis in the Fritillaria japonica group

The Fritillaria japonica group consists of six related species with x=12 and its derivative x=11. Irrespective of basic chromosome number, the pollen and embryo sac mother cells in all these species show clearly different modes of bivalent formation. Although meiosis in EMC's of five species is typically chiasmate, PMC's of all six species show achiasmate meiosis with the following characteristics: (a) Synapsis of homologous chromosomes is prolonged up to metaphase I. Separation of non-sister chromatids is suppressed and there is thus no typical diplotene/diakinesis. (b) The homologous chromosomes apposed in parallel are usually devoid of chiasmata at metaphase I. Occasionally, concealed chiasmata are observed to be hidden in the synaptic plane, (c) Separation of homologues is initiated at the kinetochore regions at early metaphase I and progresses toward the distal ends, (d) Meiotic behaviour of the small telocentric B chromosomes seems to be independent of, or incompletely controlled by, the achiasmate meiotic system. Achiasmate meiosis is assumed to have arisen from the chiasmate one through a transitional step like cryptochiasmate meiosis.