Parthenogenesis in the parasitic and free-living forms of Strongyloides papillosus (Nematoda,Rhabdiasoidea)

Both parasitic and free-living females of a calf strain of Strongyloides papillosus have a chromosome number of 2n=4. Both forms reproduce by diploid parthenogenesis. Oocytes of parasitic females undergo only one homeotypic maturation division without homologous chromosome pairing (mitotic parthenogenesis). Oocytes of free-living females show normal pairing and disjunction of the homologous chromosomes, but only one diploid polar body is expelled (meiotic parthenogenesis). Reconstitution of the diploid chromosome number occurs by separation of the two sister chromatids of each univalent during or after anaphase I.This investigation was supported by the Consiglio Nazionale delle Ricerche (C.N.R.) of Italy.     

Egg maturation and parthenogenetic recovery of diploidy in the scorpion Liocheles australasiae (Fabricius) (Scorpions, ischnuridae)

In the scorpion Liocheles australasiae, egg maturation and parthenogenetic recoveries of chromosome number and nuclear DNA content were examined by histological, karyological observations and quantitative measurements of DNA. The primary oocyte becomes mature through two successive maturation divisions. The first maturation division takes place in the primary oocyte to produce a secondary oocyte and a first polar body. The second maturation division soon occurs in the secondary oocyte, in which the nucleus is divided into a mature egg nucleus and a second polar body nucleus, not followed by cytoplasmic fission. The first polar body, in one case, was successively divided into two second polar bodies; in the other case it was not divided. In either case, these polar bodies remained attached to the early embryo. The fate of these polar bodies during further embryogenesis were studied. In the karyological analysis, the chromosome number was divided into two groups, one from 27-32, the other was 54-64. The former was presumably the metaphase chromosome number at the meiotic division; the latter was presumably the metaphase chromosome number at the mitotic division. DNA content in the diploid nucleus of the primary oocyte, doubled before the maturation divisions, was reduced through the maturation divisions by one-half in the nuclei of the secondary oocyte and the first polar body and by one-fourth in the nuclei of the egg and the second polar bodies. The first reduction of DNA content corresponded to halving the number of the chromosomes in the first maturation division and the second to the nuclear division in the secondary oocyte. These reductions represent a common process of egg maturation, except the final production of the mature egg with two haploid nuclei, an egg nucleus, and a second polar body nucleus. These two nuclei, which were formed apart in the mature egg, drew near to fuse into a zygote nucleus. The chromosome number and nuclear DNA content were doubled in the zygote and each blastomere in embryos, supporting the hypothesis that the egg nucleus fuses with the second polar body nucleus and this conjugation initiates subsequent embryonic development.     

Reproduction,reproductive organs,and meiosis in the bisexual non-parthenogenetic mite Caloglyphus mycophagus,with reference to oocyte degeneration in virgins (Sarcoptiformes: Acaridae)

Meiosis is described in virgin females, inseminated females and males of the acarid mite Caloglyphus mycophagus (Megnin). The observed sex determining mechanism is an XO-type with the male having a diploid chromosome number of 15. Oogenesis in mated females is regular. Pachytene is the earliest meiotic stage which is readily identifiable. At metaphase I eight bivalents are observed. Both products of the first maturation division divide at the second maturation division. After the fusion of the pronuclei either 15 or 16 chromosomes are observed in cleaving eggs. Nurse cells are not observed during the growth period of the oocyte. Such oocytes are attached to a central structure of the ovary by a cone-shaped organelle. At this stage the nucleus appears as a germinal vesicle; a nucleolus is present and the diffuse chromatin appears to extend from the nucleolus to the nuclear membrane. Nuclear extrusion bodies can be seen adjacent to the nuclear membrane both within and outside of the nucleus. Virgin females do not oviposit. The aberrant morphology and behavior of bivalents in post diakinetic oocytes which have not been penetrated by a sperm are described. Neither chromatin nor a chorion could be demonstrated in aberrant oocytes situated in the oviduct. It is suggested that oocyte degeneration in virgins is an adaptive feature in an animal order in which parthenogenesis is the more common mode of reproduction.     

Hypoxia inhibits fish spawning via LH-dependent final oocyte maturation

To evaluate the effects of long term hypoxia exposure on fish spawning, mature common carp, Cyprinus carpio carpio (Linnaeus) were subjected to either normoxia (7.4+/-0.2 mgO(2)mg O(2) L(-1)) or hypoxia (1.0+/-0.2 mgO(2)O(2) L(-1)) for more than two months. Gonadosomatic index (GSI), and concentrations of serum luteinizing hormone (LH), testosterone (T), and estroldiol (E2) were measured and gonad histology examined. Hypoxia inhibits fish spawning even though the gonad and oocytes developed under hypoxia exposure. LH levels of female carp were significantly decreased upon chronic exposure to hypoxia, and the final oocyte maturation in hypoxic females was significantly retarded. The results indicated that hypoxia may inhibit fish spawning through LH-dependent final oocyte maturation. In addition, no courtship was observed in hypoxic males. In conclusion, hypoxia impairs fish ovulation and, therefore, spawning and reproduction. LH levels were reduced leading to a failure of oocyte maturation. This, along with a lack of courtship by males may be the major mechanisms involved in hypoxic inhibition of reproduction in carp.     

Polyploidy and reproductive patterns in the root-knot nematode Meloidogyne hapla

A total of 32 populations and egg mass isolates of Meloidogyne hapla obtained from various geographical areas were studied cytologically and with respect to their mode of reproduction. In 29, maturation of oocytes is by regular meiosis. The reduced chromosome number at metaphase I is 17 in 18 populations, 16 in 8, and 15 in 3 populations. Reproduction in all these populations is by cross-fertilization, although nonfertilized eggs can develop by parthenogenesis. In the latter case, the two groups of telophase chromosomes of the second maturation division become enclosed in the same pronucleus, thus reestablishing the somatic chromosome number. Maturation of spermatocytes in three populations studied is by regular meiosis and the reduced chromosome number appears to be equal to that of the oocytes. In the remaining three populations, no synapsis takes place and the somatic number of 45 chromosomes is observed at metaphase of the single maturation division of both oocytes and spermatocytes. Reproduction is by obligatory mitotic parthenogensis. It is postulated that the basic chromosome number for the genus is nine and that the facultatively parthenogenetic populations are tetraploid, whereas, the obligatorily parthenogenetic populations are pentaploid. A preliminary scheme of the phylogeny in the family Heteroderidae is given.     

Meiotic parthenogenesis and heterochromatization in a soft scale,pulvinaria hydrangeae (Coccoidea: Homoptera)

Summary Meiotic parthenogenesis of a type not previously described was found in Pulvinaria hydrangeae Steinweden. During diakinesis 8 bivalents were formed. At prometaphase the spindle was tripolar but anaphase I was bipolar and normal. After completion of division of the primary oocyte, the following sequence occurred: 1. polar body I divided, usually into 3 products; 2. the secondary oocyte divided to yield the egg pronucleus and polar body II; 3. the egg pronucleus divided into its two haploid products; and 4. the second polar body divided. The products of the egg pronucleus fused while dividing to restore the diploid chromosome number; this division may be equated to the first cleavage division. The products of the polar bodies did not take part in the formation of the embryo proper or the mycetocytes.Among the embryos produced by females of two out of the three populations studied some of the embryos showed a heterochromatic chromosome set, characteristic of males in this and related families. The reproductive system of the females as well as the eggs did not contain any sperm; thus the male embryos were apparently produced parthenogenetically.The euchromatic and heterochromatic chromosome sets were genetically identical, since they both originated from the egg pronucleus by mitosis. The heterochromatization of one set but not the other might be due in part to a previous difference in their position in the cytoplasm.The females were completely homozygous yet they produced male and female embryos. Thus it appears that sex determination in the group does not depend on the segregation of genetic factors in either males or females.In addition to male and female embryos, three types of degenerating embryos were observed. It is believed that these embryos were formed by polyploid somatic cells which invaded abnormal eggs and embryos and took over development.     

Meiosis in four trisomic and one double trisomic males of the newt Pleurodeles waltlii

In the newt Pleurodeles waltlii, meiosis was studied in four trisomic and one double trisomic males. Study of first prophase shows that trivalent frequencies and trivalent configurations are correlated with chromosome length; moreover, trivalent configurations indicate that long chromosomes have multiple points of initiation of synapsis whereas two points might be adequate to secure synapsis of short chromosomes. From the study of metaphase II it appears that the extra chromosomes segregate in half of the spermatocytes II. Some abnormal spermatocytes, resulting from nondisjunction of chromosomes at mitosis or at first division of meiosis, or from precocious division of chromosomes at first division of meiosis were observed. In the male double trisomic meiosis fails at anaphase of second division; this accounts for the sterility of the individual.     

Automictic parthenogenesis of a geographically parthenogenetic mayfly,Ephoron shigae (Insecta: Ephemeroptera,Polymitarcyidae)

In the geographically parthenogenetic mayfly, Ephoron shigae, egg maturation and counts of chromosome number of unfertilized, parthenogenetic eggs were studied, in comparison with fertilized eggs from a bisexual population. The primary oocyte becomes mature through two successive maturation divisions. The first maturation division (meiotic division) takes place in the primary oocyte to produce a secondary oocyte and a first polar body. The second maturation division soon occurs in the secondary oocyte, in which the nucleus is divided into a mature egg nucleus (female pronucleus) and second polar body nuclei. The first polar body, in some cases, was successively divided into two polar bodies; in other instances, it was not divided. After the successive maturation division, the egg nucleus and the sister second polar body nucleus drew near to fuse into the zygote nucleus. The chromosome number was doubled in the zygote, and this conjugation initiates subsequent embryonic development. This suggests that, in E. shigae, the process of parthenogenetic recovery of diploidy is the automictic type categorized as the ‘terminal fusion’ pattern. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99 , 335–343.     

Diploid arrhenotoky and automictic thelytoky in soft scale insects (Lecaniidae: Coccoidea: Homoptera)

Parthenogenesis is reported in three soft scales with 2n=16. In the unfertilized eggs of all three, oogenesis is normal and diploidy is restored by the fusion of the division products of the haploid female pronucleus. In Lecanium putmani Phillips 12 of 13 uninseminated females collected in the wild produced only males. The 21 inseminated females produced 15% males. The males were diploid but contained one euchromatic (E) and one heterochromatic (H) chromosome set. Most of the eggs produced by the inseminated females contained sperm but a few did not. It was concluded, therefore, that females develop from fertilized eggs and males from unfertilized eggs and that the species was diploid arrhenotokous. In L. cerasifex Fitch only 18 of 56 females collected in the wild had been inseminated. The frequency of males among their embryos was 22%. The males were again diploid with one E and one H set of chromosomes. Among the 38 uninseminated females, 27 produced only males, and 10 produced only females. All the female producers contained needle-like bacterial symbionts. Most of the male producers, and most of the inseminated females contained no symbionts; the rest contained rod-like symbionts. It was concluded, therefore, that the females of L. cerasifex studied belonged to two races, a diploid arrhenotokous race and an obligate automictic thelytokous race. Eucalymantus tessellatus (Signoret) is obligate automictic thelytokous. All the females examined were uninseminated and produced only females.Supported by Grant GB 23665 from the National Science Foundation, Washington, D.C.     

Female reproductive biology of two sympatric incirrate octopod species, <Emphasis Type="Italic">Adelieledone polymorpha</Emphasis> (Robson 1930) and <Emphasis Type="Italic">Pareledone turqueti</Emphasis> (Joubin 1905) (Cephalopoda: Octopodidae), from South Georgia

The reproductive biology of two species of endemic Southern Ocean octopods was investigated around the sub-Antarctic islands of South Georgia and Shag Rocks. The females of both the species produce few, large eggs. This appears to be governed by phylogenetic constraint. No evidence was found for ontogenetic migration or seasonality associated with gonad maturation. Based on oocyte length frequency distributions and observations of oocyte development within the ovary, it is possible that both species could have either a single or multiple-batch spawning strategy. Pareledone turqueti ovaries contained fewer larger oocytes than those of Adelieledone polymorpha, which may help to reduce competition for resources immediately after hatching.     

Die Zytologie der Bastarde aus der Kreuzung parthenogenetischer Weibchen von Solenobia triquetrella F. R. mit Männchen der bisexuellen Rasse

Crossing of the diploid parthenogenetic form with bisexual Solenobia triquetrella males results in normal sexes in F₁ and F₂. Therefore, oogenesis and spermatogenesis of the hybrids are expected to be normal. This is the case. If diploid or tetraploid parthenogenetic females are crossed, relatively often gynanders are observed in F₁. Their appearance is explained by the assumption that in the antagonism between the zygotic nucleus (?Befruchtungskern“) and the nucleus resulting from the fusion of the second polar nucleus with the inner daughter nucleus of the first polar nucleus (?Richtungskopulationskern“ [R.K.K.]) in gynanders neither the zygotic nucleus (as during bisexual propagation) nor the R.K.K. (as during parthenogenesis) dominates and suppresses the other nucleus. In the X0 type the reduced chromosome number in the tetraploid egg is 61, in the XY type 62. The sperm nucleus carries a set of 31 Chromosomes. Therefore, the intersexual F₁ hybrid should be triploid and should have 92 or 93 chromosomes respectively. It is shown that this is indeed the case. No elimination of single chromosomes has been observed, which clearly disproves the attempt to explain the complicated mosaic of the intersexes by the assumption that the 3A∶2X ratio changes during development. In all cases tested chromosome conjugation must have been complete, since in metaphase I the egg always shows 31 trivalents. (Spermatogenesis will be described in Mitteilung II.) Each autosomal trivalent consists of 3 homologous chromosomes, 2 of which are derived from the mother and the third from the sperm. During the reductional division the two maternally derived homologues seem always to go to opposite poles, while the paternal chromosome migrates to either one; therefore the daughter in the XY type must have a distribution of, for example, 42∶51=93, in the X0 type of, for example, 45∶47=92. It is proved that this is the case. The second maturation division in the egg is normal. All chromosomes are divided equationally; thus the mature egg nucleus contains a variable number of chromosomes. Further propagation of predominantly female intersexes is possible by parthenogenesis. Since during parthenogenesis the organism is derived from the R.K.K. the triploid chromosome number remains unchanged as is demonstrated. The parthenogenetic propagation of F₁ eggs thus must lead to the same result as in F₁. It is made highly probable that this is the case.     

哺乳动物卵母细胞不对称分裂的研究进展

哺乳动物卵母细胞成熟过程需要进行两次连续的不对称分裂,最终形成体积差异巨大的子细胞：大体积的卵母细胞和两种体积较小的极体。不对称分裂现象是哺乳动物卵母细胞减数分裂的典型特征,不对称分裂后的卵母细胞是高度极化的细胞。精卵结合后,细胞重新恢复了对称分裂,但是在卵母细胞减数分裂过程中形成的极性特征却得以保留并影响早期胚胎的极性。本文对近年来在哺乳动物卵母细胞不对称分裂方面的相关研究展开综述,从细胞质不对称分裂和细胞核不对称分裂两个方面对染色体、细胞骨架在哺乳动物卵母细胞不对称分裂中的作用、细胞器在哺乳动物卵母细胞成熟过程中的重组分配、染色体非随机分离等过程进行介绍,旨在从细胞和分子水平阐述哺乳动物卵母细胞不对称分裂的主要机制。     

Evidence for a coelomic maturation factor controlling oocyte maturation in the polychaete Arenicola marina (L.)

Summary

Previous studies on Arenicola marina suggested that oocyte maturation was induced by a single maturation hormone from the prostomium. This maturation hormone was thought to act directly on the oocyte (Meijer and Durchon, 1977), A recently described species, Arenicola defodiens (Cadman and Nelson-Smith, 1993), morphologically very similar to A. marina, has been found at the sampling sites described by Meijer and Durchon (1977). Results presented here from studies on British populations of Arenicola marina show that in this species, oocyte maturation is controlled by two hormonal steps. The first step involves the prostomial maturation hormone. The second step depends on a maturation inducing substance in the coelomic fluid. We will refer to this as the coelomic maturation factor (CMF). A reliable in vitro assay for oocyte maturation in the lugworm Arenicola marina has been adopted. It utilizes fluorescence staining of the chromosome material with DNA labelling dyes (Hoechst 33342 and 33258). Maturation of oocytes in A. marina involves germinal vesicle breakdown (GVBD). This is accompanied by the movement of chromosomes from late prophase to metaphase of meiosis I and chromosome condensation. The chromosomes are stained brightly by the dyes and their relative positions can be easily identified so that mature and immature eggs can be distinguished by the differences in chromosome position and form. The development of the in vitro fluorescence assay has enabled us to demonstrate that there are two endocrine steps involved in the induction of oocyte maturation. We have begun the characterization of CMF, and data show this to be a thermolabile molecule with a molecular mass greater than 10 kd.     

Biased Transmission of Sex Chromosomes in the Aphid Myzus persicae Is Not Associated with Reproductive Mode

Commonly, a single aphid species exhibits a wide range of reproductive strategies including cyclical parthenogenesis and obligate parthenogenesis. Sex determination in aphids is chromosomal; females have two X chromosomes, while males have one. X chromosome elimination at male production is generally random, resulting in equal representation of both X chromosomes in sons. However, two studies have demonstrated deviations from randomness in some lineages. One hypothesis to account for such deviations is that recessive deleterious mutations accumulate during bouts of asexual reproduction and affect male viability, resulting in overrepresentation of males with the least deleterious of the two maternal X chromosomes. This hypothesis results in a testable prediction: X chromosome transmission bias will increase with time spent in the asexual phase and should therefore be most extreme in the least sexual aphid life cycle class. Here we test this prediction in Myzus persicae. We used multiple heterozygous X-linked microsatellite markers to screen 1085 males from 95 lines of known life cycle. We found significant deviations from equal representation of X chromosomes in 15 lines; however, these lines included representatives of all life cycles. Our results are inconsistent with the hypothesis that deviations from randomness are attributable to mutation accumulation.     

Gametogenesis and fertilization in Rhabdias ranae Walton 1929: I. The parasitic hermaphrodite

The somatic, diploid, and haploid chromosome numbers were determined in the hermaphrodite (24, 12, eggs 6, spermatozoa 5 or 6). Oocytes in the ovary synapsis zone contain 6 tetrads. The tetrads seem to disappear in the growth zone and reappear in the oocyte just prior to fertilization. After sperm penetration, the tetrads move to the periphery and form 2 polocytes by rapid meiotic divisions. Spermatogenesis occurs in the testis zone. Primary spermatocytes show 5 large tetrads and 2 small heterochromosomal dyads which lag on the spindle during division. Each resultant secondary spermatocyte has 1 small and 5 large dyads. In the equational division, one of the lagging heterochromosomes (X) is lost in a small cytophore as cytokinesis occurs. Two spermatids are produced, one with 5 chromosomes and the other with 6 chromosomes. Fertilization occurs in the seminal receptacle and results in either free-living males (2N = 11) or females (2N = 12).     

Ultrastructural and cytochemical studies on the germarium of Dicrocoelium dendriticum (Plathelminthes,Digenea)

Summary The unpaired germarium of Dicrocoelium dendriticum contains many female germ cells at different stages of maturation and is enveloped by a fibrous basal lamina-like structure and a multilayered cytoplasmic sheath whose origins and functions are discussed. The maturation process of primary oocytes occurs completely within the prophase of the first meiotic division. It has been divided into three stages, as previously suggested for monogeneans. Stage I corresponds to oogonia and early oocytes which are located in the distal germinative area of the gonad. These cells are characterized by a high nucleo/cytoplasmic ratio and a poorly differentiated cytoplasm. Stage II corresponds to maturing oocytes grouped in the central area of the gonad and exhibiting long synaptonemal complexes and a prominent nucleolus. The main feature of cytoplasmic differentiation is the increase in the number of RER and Golgi complex which are involved in the production of small electron-dense granules. Stage III corresponds to mature oocytes located in the proximal area of the germarium near the origin of the oviduct. In this stage, the granules become regularly distributed in a monolayer in the peripheral ooplasm and make contact with the oolemma. They show a distinctive complex structure, are composed of proteins and glycoproteins and do not contain polyphenols. Their possible role as cortical granules is discussed in relation to chemical composition and previous studies on other Plathelminthes. Neither yolk globules nor glycogen are present in the oocytes.Abbreviations I oogonium and early oocyte - II growing oocyte - III mature oocyte - cg cortical granule - cs cytoplasmic sheath - db dense body - ecm extra cellular matrix - ER endoplasmic reticulum - fl fibrous extracellular layer - gc Golgi complex - m mitochondria - N nucleus - nu nucleolus - RER rough endoplasmic reticulum - sc synaptonemal complex     

The distribution of oocyte 5S,somatic 5S and 18S + 28S rDNA sequences in the lampbrush chromosomes of Xenopus laevis

The nucleolus organizer locus of Xenopus laevis lampbrush chromosomes was identified by in situ hybridization of a ³H-labelled probe complementary to 18S + 28S rDNA. The nucleolus organizer is an axial granule on chromosome III that lies four-fifths the way down this chromosome reading from its larger (left) telomere, just within an exploded region that extends to its right end, where the lateral loops are exceptionally long. By in situ hybridization of ³H-labelled oocyte and somatic 5S spacer cRNA probes to similarly RNase-treated and denatured lampbrush chromosomes, the multiple sites of oocyte and somatic 5S gene families were identified. Oocyte 5S genes lie at the larger telomeres of the 15 chromosomes that possess these structures; that is, all but chromosomes X, XVII and XVIII. There are a further four sites, all peripheral, and in three of these, on chromosomes VII, X and XI, the sequences lie on lateral loops that are resolvable with the light microscope. By contrast all of the somatic 5S gene clusters occupy peripheral sites. There are two sites on chromosome III, one of which may be shared with oocyte 5S sequences; one on chromosome VII, which is very likely shared with oocyte 5S sequences; one terminal site on chromosome X; one site on chromosome XI that lies on a single pair of long loops which are inserted in a conspicuous and recognizable axial granule, loops which certainly carry oocyte 5S sequences too; two nearly terminal sites alongside the larger telomeres on chromosomes XII and XIV; and single interstitial sites on all three of the sphere-bearing chromosomes, VIII, IX and XVI. We suggest that 5S sequences on resolvable loops are transcribed by readthrough from upstream promoters, probably by polymerase II.     

Aberrant spermatogenesis and the peculiar mechanism of sex determination in Symphypleonan Collembola (Insecta)

Light and electron microscopy evidence have been obtained to describe the peculiar spermatogenesis in the collembolan species Sminthurus viridis and Allacma fusca (Sminthuridae). In these two species, the two sexes differ for the lack of two chromosomes (the sex chromosomes) in males (males, 2n = 10; females, 2n = 12). While oogenesis seems to proceed normally, spermatogenesis is peculiar because the two daughter cells of the first meiotic division have different chromosome numbers (six and four). The cell receiving four chromosomes degenerates, while the cell receiving six chromosomes completes meiosis and produces identical spermatozoa (n = 6). At fertilization, pronuclei with six chromosomes fuse together to form zygotes with 2n = 12. Male embryos must lose two sex chromosomes during the first zygotic mitosis, as all male cells have 2n = 10 chromosomes. The sex chromosome system of these species can be identified as X1X1X2X2:X1X20. Electron microscopy observations show that the same peculiar spermatogenesis occurs also in two others species of the same family, Caprainea marginata and Lipothrix lubbocki. The peculiar sex determination system described is similar but not identical to what is observed in other insect orders, and it may represent an evolutionary step toward parthenogenesis. It is suggested that this peculiar spermatogenesis is common to all Symphypleona.     

Effects of deletions on mitotic stability of the Paternal-Sex-Ratio (PSR) chromosome from Nasonia

Paternal-Sex-Ratio (PSR) is a B chromosome that causes all-male offspring in the parasitoid wasp Nasonia vitripennis. It is only transmitted via sperm of carrier males and destroys the other paternal chromosomes during the first mitotic division of the fertilized egg. Because of haplodiploidy, the effect of PSR is to convert diploid (female) eggs into haploid eggs that develop into PSR-bearing males. The PSR chromosome was previously found to contain several families of repetitive DNA, which appear to be present in local blocks. PSR chromosomes with irradiation-induced deletions have decreased rates of transmission and increased variation in transmission. This study investigates whether these differences in transmission of deletion chromosomes are due to mitotic instability. Two deleton chromosomes (E306 and F316) and the wild-type PSR chromosome were examined. A cytogenetic assay of testes revealed that wild-type PSR males contained the chromosome in 98%–100% of their spermatocytes. Similar counts from carriers of two delection chromosomes were lower and varied between individuals from 50%–100%. One F316 male did not contain the chromosome in any of its spermatocytes although the chromosome was present in somatic tissues based on hybridization to PSR-specific repetitive DNA. A molecular analysis of males found the wild-type PSR chromosome to be present in all somatic tissues. Tissue specific differences in the presence of PSR were found in several males from the two deletion lines. The results show that deletions can result in mosaicism due to increased mitotic instability of PSR. Such individuals sometimes partially or completely fail to transmit the chromosome. Patterns of mosaicism of B chromosomes in other organisms are discussed.by P.B. Moens