The Origin of the Achiasmatic XY Sex Chromosome System in Cacopsylla peregrina (Frst.) (Psylloidea, Homoptera)

The status of an extra univalent, if it is a B chromosome or an achiasmatic Y chromosome, associating with the X chromosome in male meiosis of Cacopsylla peregrina (Frst.) (Homoptera, Psylloidea) was analysed. One extra univalent was present in all males collected from three geographically well separated populations, it was mitotically stable, and showed precise segregation from the X chromosome. These findings led us to propose that the univalent represents in fact a Y chromosome. The behaviour of the X and Y chromosomes during meiotic prophase suggested that their regular segregation was based on an achiasmatic segregation mechanism characterised by a 'touch and go' pairing of segregating chromosomes at metaphase I. To explain the formation of the achiasmatic Y within an insect group with X0 sex chromosome system, it was suggested that the Y chromosome has evolved from a mitotically stable B chromosome that was first integrated into an achiasmatic segregation system with the X chromosome, and has later become fixed in the karyotype as a Y chromosome.     

Interaction of B chromosomes with A or B chromosomes in segregation in insects

Additional or B chromosomes not belonging to the regular karyotype of a species are found in many animal and plant groups. They form a highly heterogeneous group with respect to their morphology and behaviour both in mitosis and meiosis. Achiasmatic mechanisms that ensure the segregation of a B chromosome from another B chromosome or from an A chromosome are reviewed. An achiasmatic mechanism characterized by the "distance pairing" of segregating univalents at metaphase I was found to be responsible for the preferential segregation of B chromosome univalents in Hemerobius marginatus L. (Neuroptera), and a mechanism characterized by the "touch and go pairing" of segregating univalents was responsible for the highly regular segregation of a B chromosome and the X chromosome in Rhinocola aceris (L.) (Psylloidea, Homoptera). The latter mechanism resulted in the integration of a B chromosome to the A chromosome set as a Y chromosome in a psyllid species Cacopsylla peregrina (Frst.). Furthermore, B chromosomes can disturb the regular segregation of the achiasmatic X and Y chromosomes resulting in the formation of X0/XY polymorphism in a population, which might precede the loss of the Y chromosome. The absence of observations on accurately functioning achiasmatic segregation mechanisms in grasshoppers (Orthoptera) was attributed to the X and B chromosomes, which re-orient one or several times during metaphase I. Apparently, these re-orientations mask any achiasmatic segregation mechanism that might operate during meiotic prophase in these insects.     

A deficiency screen of the major autosomes identifies a gene (matrimony) that is haplo-insufficient for achiasmate segregation in Drosophila oocytes

In Drosophila oocytes, euchromatic homolog-homolog associations are released at the end of pachytene, while heterochromatic pairings persist until metaphase I. A screen of 123 autosomal deficiencies for dominant effects on achiasmate chromosome segregation has identified a single gene that is haplo-insufficient for homologous achiasmate segregation and whose product may be required for the maintenance of such heterochromatic pairings. Of the deficiencies tested, only one exhibited a strong dominant effect on achiasmate segregation, inducing both X and fourth chromosome nondisjunction in FM7/X females. Five overlapping deficiencies showed a similar dominant effect on achiasmate chromosome disjunction and mapped the haplo-insufficient meiotic gene to a small interval within 66C7-12. A P-element insertion mutation in this interval exhibits a similar dominant effect on achiasmate segregation, inducing both high levels of X and fourth chromosome nondisjunction in FM7/X females and high levels of fourth chromosome nondisjunction in X/X females. The insertion site for this P element lies immediately upstream of CG18543, and germline expression of a UAS-CG18543 cDNA construct driven by nanos-GAL4 fully rescues the dominant meiotic defect. We conclude that CG18543 is the haplo-insufficient gene and have renamed this gene matrimony (mtrm). Cytological studies of prometaphase and metaphase I in mtrm hemizygotes demonstrate that achiasmate chromosomes are not properly positioned with respect to their homolog on the meiotic spindle. One possible, albeit speculative, interpretation of these data is that the presence of only a single copy of mtrm disrupts the function of whatever "glue" holds heterochromatically paired homologs together from the end of pachytene until metaphase I.     

There are two mechanisms of achiasmate segregation in Drosophila females,one of which requires heterochromatic homology

There are numerous examples of the regular segregation of achiasmate chromosomes at meiosis I in Drosophila melanogaster females. Classically, the choice of achiasmate segregational partners has been thought to be independent of homology, but rather made on the basis of availability or similarities in size and shape. To the contrary, we show here that heterochromatic homology plays a primary role in ensuring the proper segregation of achiasmate homologs. We observe that the heterochromatin of chromosome 4 functions as, or contains, a meiotic pairing site. We show that free duplications carrying the 4th chromosome pericentric heterochromatin induce high frequencies of 4th chromosome nondisjunction regardless of their size. Moreover, a duplication from which some of the 4th chromosome heterochromatin has been removed is unable to induce 4th chromosome nondisjunction. Similarly, in the absence of either euchromatic homology or a size similarity, duplications bearing the X chromosome heterochromatin also disrupt the segregation of two achiasmate X chromosome centromeres. Although heterochromatic regions are sufficient to conjoin nonexchange homologues, we confirm that the segregation of heterologous chromosomes is determined by size, shape, and availability. The meiotic mutation Axs differentiates between these two processes of achiasmate centromere coorientation by disrupting only the homology-dependent mechanism. Thus there are two different mechanisms by which achiasmate segregational partners are chosen. We propose that the absence of diplotene-diakinesis during female meiosis allows heterochromatic pairings to persist until prometaphase and thus to co-orient homologous centromeres. We also propose that heterologous disjunctions result from a separate and homology-independent process that likely occurs during prometaphase. The latter process, which may not require the physical association of segregational partners, is similar to those observed in many insects, in Saccharomyces cerevisiae and in C. elegans males. We also suggest that the physical basis of this process may reflect known properties of the Drosophila meiotic spindle. © 1993 Wiley-Liss, Inc.     

Chromosomal segregational mechanisms in ant-lions (Myrmeleontidae,Neuroptera)

In a single male specimen of Myrmeleon mexicanum Banks the sex chromosomes, normally X and Y, were replaced by what appeared to be X¹X² and Y. These segregated as expected on that interpretation in only half of the spermatocytes — in the other half, one X and the Y segregated from the other X. This atypical segregation is explicable on the assumption that one of the supposed Xs is a supernumerary, not a sex chromosome, and the diploid complement of the male comprises six pairs of autosomes plus a supernumerary and the X and Y sex chromosomes. The orientation of the X chromosomes at first metaphase was variable: kinetochoric activity may be localized midway the length of the chromosome, as in gonial mitosis, or terminally. Comparative study of three congeneric species, seven of Brachynemurus, one of Psammoleon, and one of Vella showed normal segregation in all, and no evidence for secondary kinetochoric activity. In nine of the species studied one pair of autosomes was unconjoined at first metaphase in 0.3%–1.2% of primary spermatocytes. These autosomes segregated precociously with the sex chromosomes in the central unit of the spindle. In one exceptional male of Brachynemurus hubbardi Currie all first meiotic metaphases showed this behavior, and a compound X¹X²/Y¹Y² system was thus simulated. Bivalent formation replaced distance segregation of sex chromosomes in 0.4%–3.2% of the spermatocytes in seven of the thirteen species studied. These sex-bivalents frequently displayed partial or complete failure in congression.     

Heterochromatic Threads Connect Oscillating Chromosomes during Prometaphase I in Drosophila Oocytes

In Drosophila oocytes achiasmate homologs are faithfully segregated to opposite poles at meiosis I via a process referred to as achiasmate homologous segregation. We observed that achiasmate homologs display dynamic movements on the meiotic spindle during mid-prometaphase. An analysis of living prometaphase oocytes revealed both the rejoining of achiasmate X chromosomes initially located on opposite half-spindles and the separation toward opposite poles of two X chromosomes that were initially located on the same half spindle. When the two achiasmate X chromosomes were positioned on opposite halves of the spindle their kinetochores appeared to display proper co-orientation. However, when both Xs were located on the same half spindle their kinetochores appeared to be oriented in the same direction. Thus, the prometaphase movement of achiasmate chromosomes is a congression-like process in which the two homologs undergo both separation and rejoining events that result in the either loss or establishment of proper kinetochore co-orientation. During this period of dynamic chromosome movement, the achiasmate homologs were connected by heterochromatic threads that can span large distances relative to the length of the developing spindle. Additionally, the passenger complex proteins Incenp and Aurora B appeared to localize to these heterochromatic threads. We propose that these threads assist in the rejoining of homologs and the congression of the migrating achiasmate homologs back to the main chromosomal mass prior to metaphase arrest.     

Linkage of phosphoribosylpyrophosphate synthetases 1 and 2, Prps1 and Prps2, on the mouse X Chromosome

The X Chromosome (Chr) genes for phosphoribosylpyrophosphate synthetases 1 and 2, Prps1 and Prps2, were mapped on the mouse X Chr with interspecific backcrosses between C57BL/6 (B6) and M. spretus (S). Southern analysis showed that Prps1 mapped between Plp and DXWas31, a mouse X Chr region that is homologous to Xq21-24 on the human X Chr while Prps2 mapped between DXWas31 and Amg, a region that is homologous to the map position of PRPS2 on Xp22 of the human X Chr. Additionally, other restriction fragments highlighted by PRS II showed autosomal segregation. In situ hybridization and FISH analysis of metaphase chromosome spreads prepared from lymphocytes of B6 or S male mice confirmed that there were in fact two different locations on the X Chr, X F1-2 and X F2-3 for Prps1 and 2 respectively, as well as two autosomal sites for Prps-like genes.     

The meiotic defects of mutants in the Drosophila mps1 gene reveal a critical role of Mps1 in the segregation of achiasmate homologs

The conserved kinase Mps1 is necessary for the proper functioning of the mitotic and meiotic spindle checkpoints (MSCs), which monitor the integrity of the spindle apparatus and prevent cells from progressing into anaphase until chromosomes are properly aligned on the metaphase plate. In Drosophila melanogaster, a null allele of the gene encoding Mps1 was recently shown to be required for the proper functioning of the MSC, but it did not appear to exhibit a defect in female meiosis. We demonstrate here that the meiotic mutant ald1 is a hypomorphic allele of the mps1 gene. Both ald1 and a P-insertion allele of mps1 exhibit defects in female meiotic chromosome segregation. The observed segregational defects are substantially more severe for pairs of achiasmate homologs, which are normally segregated by the achiasmate (or distributive) segregation system, than they are for chiasmate bivalents. Furthermore, cytological analysis of ald1 mutant oocytes reveals both a failure in the coorientation of achiasmate homologs at metaphase I and a defect in the maintenance of the chiasmate homolog associations that are normally observed at metaphase I. We conclude that Mps1 plays an important role in Drosophila female meiosis by regulating processes that are especially critical for ensuring the proper segregation of nonexchange chromosomes.     

Chromosome micromanipulation

The relationship of kinetochore orientation and reorientation to orderly chromosome distribution in anaphase has been studied experimentally by micromanipulation of living grasshopper spermatocytes. Bivalents or the X chromosome at prometaphase or metaphase I can be detached from the spindle with a microneedle and moved to any desired location within the cell. Following a pause of variable duration the detached chromosome invariably moved, kinetochores foremost, back to the spindle, reassumed its characteristic metaphase position, and, with one exception, segregated normally at anaphase I. Detachment from the spindle is demonstrated unequivocally (1) by manipulation evidence for the absence of the firm spindle connections seen both before detachment and after reattachment and (2) by a functional criterion: a given kinetochore, oriented to one pole before detachment, often orients to the opposite pole after detachment. The segregation in anaphase was always as expected from the final, post-operation, orientation. Reorientation and prometaphase and anaphase movement after detachment cannot be distinguished from their counterparts in control cells. Kinetochore position after detachment is the primary determinant of the pole to which that kinetochore will orient. Therefore, since the experimenter determines kinetochore position, he can cause any given half-bivalent to segregate to a predetermined pole at anaphase I. Similarly, orientation of both half-bivalents to the same pole can be induced. These mal-oriented bivalents invariably reorient and normal anaphase segregation ensues. Non-disjunction can, however, be produced directly in late anaphase. These experiments are based upon current views of orderly chromosome distribution; their success confirms our understanding of the fundamental orientation process.     

Estimating Tempo and Mode of Y Chromosome Turnover: Explaining Y Chromosome Loss With the Fragile Y Hypothesis

Chromosomal sex determination is phylogenetically widespread, having arisen independently in many lineages. Decades of theoretical work provide predictions about sex chromosome differentiation that are well supported by observations in both XY and ZW systems. However, the phylogenetic scope of previous work gives us a limited understanding of the pace of sex chromosome gain and loss and why Y or W chromosomes are more often lost in some lineages than others, creating XO or ZO systems. To gain phylogenetic breadth we therefore assembled a database of 4724 beetle species’ karyotypes and found substantial variation in sex chromosome systems. We used the data to estimate rates of Y chromosome gain and loss across a phylogeny of 1126 taxa estimated from seven genes. Contrary to our initial expectations, we find that highly degenerated Y chromosomes of many members of the suborder Polyphaga are rarely lost, and that cases of Y chromosome loss are strongly associated with chiasmatic segregation during male meiosis. We propose the “fragile Y” hypothesis, that recurrent selection to reduce recombination between the X and Y chromosome leads to the evolution of a small pseudoautosomal region (PAR), which, in taxa that require XY chiasmata for proper segregation during meiosis, increases the probability of aneuploid gamete production, with Y chromosome loss. This hypothesis predicts that taxa that evolve achiasmatic segregation during male meiosis will rarely lose the Y chromosome. We discuss data from mammals, which are consistent with our prediction.     

Chromosome relationships and meiotic mechanisms of certain morsitans group tsetse flies and their hybrids

There are three pairs of euchromatic components, the L₁ and L₂ autosomes and the X chromosome pair respectively, which are found in both G. austeni and the three forms of G. morsitans. Each species/sub-species also includes in its complement a group of heterochromatic autosomes (S) which have various morphologies and differ in number both within and between the species/sub-species. Several lines of evidence are outlined which point to these being supernumerary B chromosomes. Male meiosis is normally achiasmate and only L₁ and L₂ autosomes pair completely. X-Y association is restricted to a small pairing segment the position of which on the X is constant for all the species/sub-species. It is located in one of two positions on the Y chromosome according to the species/sub-species. The S chromosomes behave as hereditary univalents at first anaphase while the sex bivalent can undergo distance pairing best exemplified in G. austeni and G. submorsitans. A Y structural mutant line gives some indication of the size of the pairing segment and demonstrates that survival and maleness is possible even when two-thirds of the chromosome is missing. Meiotic and polytene chromosome studies connected with hybridisation experiments designed to test the sterility factor as a potential means of tsetse control assist in establishing the evolutionary relationship of the subspecies.     

The mechanism of secondary nondisjunction in Drosophila melanogaster females

Bridges (1916) observed that X chromosome nondisjunction was much more frequent in XXY females than it was in genetically normal XX females. In addition, virtually all cases of X nondisjunction in XXY females were due to XX <--> Y segregational events in oocytes in which the two X chromosomes had failed to undergo crossing over. He referred to these XX <--> Y segregation events as "secondary nondisjunction." Cooper (1948) proposed that secondary nondisjunction results from the formation of an X-Y-X trivalent, such that the Y chromosome directs the segregation of two achiasmate X chromosomes to opposite poles on the first meiotic spindle. Using in situ hybridization to X and YL chromosomal satellite sequences, we demonstrate that XX <--> Y segregations are indeed presaged by physical associations of the X and Y chromosomal heterochromatin. The physical colocalization of the three sex chromosomes is observed in virtually all oocytes in early prophase and maintained at high frequency until midprophase in all genotypes examined. Although these XXY associations are usually dissolved by late prophase in oocytes that undergo X chromosomal crossing over, they are maintained throughout prophase in oocytes with nonexchange X chromosomes. The persistence of such XXY associations in the absence of exchange presumably facilitates the segregation of the two X chromosomes and the Y chromosome to opposite poles on the developing meiotic spindle. Moreover, the observation that XXY pairings are dissolved at the end of pachytene in oocytes that do undergo X chromosomal crossing over demonstrates that exchanges can alter heterochromatic (and thus presumably centromeric) associations during meiotic prophase.     

There are two mechanisms of achiasmate segregation in Drosophila females, one of which requires heterochromatic homology.

There are numerous examples of the regular segregation of achiasmate chromosomes at meiosis I in Drosophila melanogaster females. Classically, the choice of achiasmate segregational partners has been thought to be independent of homology, but rather made on the basis of availability or similarities in size and shape. To the contrary, we show here that heterochromatic homology plays a primary role in ensuring the proper segregation of achiasmate homologs. We observe that the heterochromatin of chromosome 4 functions as, or contains, a meiotic pairing site. We show that free duplications carrying the 4th chromosome pericentric heterochromatin induce high frequencies of 4th chromosome nondisjunction regardless of their size. Moreover, a duplication from which some of the 4th chromosome heterochromatin has been removed is unable to induce 4th chromosome nondisjunction. Similarly, in the absence of either euchromatic homology or a size similarity, duplications bearing the X chromosome heterochromatin also disrupt the segregation of two achiasmate X chromosome centromeres. Although heterochromatic regions are sufficient to conjoin nonexchange homologues, we confirm that the segregation of heterologous chromosomes is determined by size, shape, and availability. The meiotic mutation Axs differentiates between these two processes of achiasmate centromere coorientation by disrupting only the homology-dependent mechanism. Thus there are two different mechanisms by which achiasmate segregational partners are chosen. We propose that the absence of diplotene-diakinesis during female meiosis allows heterochromatic pairings to persist until prometaphase and thus to co-orient homologous centromeres. We also propose that heterologous disjunctions result from a separate and homology-independent process that likely occurs during prometaphase. The latter process, which may not require the physical association of segregational partners, is similar to those observed in many insects, in Saccharomyces cerevisiae and in C. elegans males. We also suggest that the physical basis of this process may reflect known properties of the Drosophila meiotic spindle.     

Y染色体异常29例分析

本文从1992例遗传咨询病例中收集29例Y染色体异常的病例,其中Y染色体数 目异常(47,XYY)2例;Y染色体结构异常8例:Y/Y易位1例、Yp+3例、de l(Y)3例、嵌合 体dic(Y)1例;Y染色体长度变异19例。对Y染色体这几种异常类型的遗传效应进行分析。 Abstract:Twenty nine cases of Y chromosome abnormalities were found in 1992 patients asking genetic counseling.Different kinds of Y chromosome abnormalitics were detected by G and banding techniques.These were 47,XYY(2 cascs);46,X,del(Y)(3 cascs);46,X,Yp+(3 cases);46,X,t(Y;Y)(1 case);45,X/46,X,dic(Y)(1 case) and length changes of Y chromosome(19 cases).The genetic effects of Y chromosome abnormalities have been analyzed in this report.     

Common Mechanisms of Y Chromosome Evolution

Y chromosome evolution is characterized by the expansion of genetic inertness along the Y chromosome and changes in the chromosome structure, especially the tendency of becoming heterochromatic. It is generally assumed that the sex chromosome pair has developed from a pair of homologues. In an evolutionary process the proto-Y-chromosome, with a very short differential segment, develops in its final stage into a completely heterochromatic and to a great extends genetically eroded Y chromosome. The constraints evolving the Y chromosome have been the objects of speculation since the discovery of sex chromosomes. Several models have been suggested. We use the exceptional situation of the in Drosophila mirandato analyze the molecular process in progress involved in Y chromosome evolution. We suggest that the first steps in the switch from a euchromatic proto-Y-chromosome into a completely heterochromatic Y chromosome are driven by the accumulation of transposable elements, especially retrotransposons inserted along the evolving nonrecombining part of the Y chromosome. In this evolutionary process trapping and accumulation of retrotransposons on the proto-Y-chromosome should lead to conformational changes that are responsible for successive silencing of euchromatic genes, both intact or already mutated ones and eventually transform functionally euchromatic domains into genetically inert heterochromatin. Accumulation of further mutations, deletions, and duplications followed by the evolution and expansion of tandem repetitive sequence motifs of high copy number (satellite sequences) together with a few vital genes for male fertility will then represent the final state of the degenerated Y chromosome. This revised version was published online in July 2006 with corrections to the Cover Date.     

Sex Chromosomal Homologies between Human and Fish Karyotypes Revealed by Chromosome Painting

Human sex chromosome-specific probes were hybridized to metaphase spreads of three fish species, Monopterus albus Zuiew, Danio rerioandMastacembelus aculeatusBasilewsky, to reveal evolutionary conservation of sex chromosomal segments between distantly related species of vertebrates. The human X chromosomal paint disclosed 4, 8, and 6 conserved syntenic segments in the karyotypes of the three fish species respectively, which were scattered in several pairs of homologous chromosomes. But no conserved segment was identified in our experiments when the human Y chromosomal probes were used. The evolution of the X chromosome of vertebrates is discussed.     

G-, R- and C-band patterns of goral (<Emphasis Type="Italic">Nemorhaedus caudatus</Emphasis>) and comparison to goat (<Emphasis Type="Italic">Capra hircus</Emphasis>)

The karyotype of goral (Nemorhaedus caudatus, 2n = 56) was prepared using lymphocytes and its chromosomal band patterns were compared with those of goat (Capra hircus, 2n = 60) by CBG-, GTG- and RBG-banding techniques. The standard karyotype of goral was composed of 54 acrocentric autosomes, submetacentric X chromosome, and acrocentric Y chromosome. C-bands were prominent in all autosomes except the X chromosome. G- and R-band patterns of goral were dissimilar to those of goat. The data support the idea that the goral did not originate from a common ancestor of bovid, or that there were numerous complicated chromosomal interchanges during goral evolution, in contrast to other bovids.     

A sex‐linked SCAR marker in Bryonia dioica (Cucurbitaceae), a dioecious species with XY sex‐determination and homomorphic sex chromosomes

Genetic crosses between the dioecious Bryonia dioica (Cucurbitaceae) and the monoecious B. alba in 1903 provided the first clear evidence for Mendelian inheritance of dioecy and made B. dioica the first organism for which XY sex‐determination was experimentally proven. Applying molecular tools to this system, we developed a sex‐linked sequence‐characterized amplified region (SCAR) marker for B. dioica and sequenced it for individuals representing the full geographic range of the species from Scotland to North Africa. For comparison, we also sequenced this marker for representatives of the dioecious B. cretica, B. multiflora and B. syriaca, and monoecious B. alba. In no case did any individual, male or female, yield more than two haplotypes. In northern Europe, we found strong linkage between our marker and sex, with all Y‐sequences being identical to each other. In southern Europe, however, the linkage between our marker and sex was weak, with recombination detected within both the X‐ and the Y‐homologues. Population genetic analyses suggest that the SCAR marker experienced different evolutionary pressures in northern and southern Europe. These findings fit with phylogenetic evidence that the XY system in Bryonia is labile and suggest that the genus may be a good system in which to study the early steps of sex chromosome evolution.     

Species-Specific Heterochromatin Prevents Mitotic Chromosome Segregation to Cause Hybrid Lethality in Drosophila

Postzygotic reproductive barriers such as sterility and lethality of hybrids are important for establishing and maintaining reproductive isolation between species. Identifying the causal loci and discerning how they interfere with the development of hybrids is essential for understanding how hybrid incompatibilities (HIs) evolve, but little is known about the mechanisms of how HI genes cause hybrid dysfunctions. A previously discovered Drosophila melanogaster locus called Zhr causes lethality in F1 daughters from crosses between Drosophila simulans females and D. melanogaster males. Zhr maps to a heterochromatic region of the D. melanogaster X that contains 359-bp satellite repeats, suggesting either that Zhr is a rare protein-coding gene embedded within heterochromatin, or is a locus consisting of the noncoding repetitive DNA that forms heterochromatin. The latter possibility raises the question of how heterochromatic DNA can induce lethality in hybrids. Here we show that hybrid females die because of widespread mitotic defects induced by lagging chromatin at the time during early embryogenesis when heterochromatin is first established. The lagging chromatin is confined solely to the paternally inherited D. melanogaster X chromatids, and consists predominantly of DNA from the 359-bp satellite block. We further found that a rearranged X chromosome carrying a deletion of the entire 359-bp satellite block segregated normally, while a translocation of the 359-bp satellite block to the Y chromosome resulted in defective Y segregation in males, strongly suggesting that the 359-bp satellite block specifically and directly inhibits chromatid separation. In hybrids produced from wild-type parents, the 359-bp satellite block was highly stretched and abnormally enriched with Topoisomerase II throughout mitosis. The 359-bp satellite block is not present in D. simulans, suggesting that lethality is caused by the absence or divergence of factors in the D. simulans maternal cytoplasm that are required for heterochromatin formation of this species-specific satellite block. These findings demonstrate how divergence of noncoding repetitive sequences between species can directly cause reproductive isolation by altering chromosome segregation.     

Cytomorphological study of Scutellaria platystegia (Lamiaceae) in Iran

Chromosome number, meiotic behaviour and morphological characters related to habit were studied in 10 populations of Scutellaria platystegia Juz. from S. sect. Lupulinaria native to Iran. All populations are diploid and has the chromosome number 2n = 2x = 22, which is consistent with the proposed base number of x = 11. This taxon displayed regular bivalent pairing and chromosome segregation at meiosis. However, some meiotic abnormalities observed included various degrees of fragmented chromosomes, laggards and bridges in anaphase I to telophase II, precocious division of centromeres in metaphase I or II, asynchronous nucleus and cytomixis. We evaluated and determined the population limits within S. platystegia, employing multivariant statistics. We found a striking association between meiotic behaviour and gross morphology. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)