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The cell division cycle involves nuclear and cytoplasmic events, namely organelle multiplication and distribution between the daughter cells. Until now, plastid and plant cell division have been considered as independent processes because they can be uncoupled. Here, down-regulation of AtCDT1a and AtCDT1b, members of the prereplication complex, is shown to alter both nuclear DNA replication and plastid division in Arabidopsis thaliana . These data constitute molecular evidence for relationships between the cell-cycle and plastid division. Moreover, the severe developmental defects observed in AtCDT1-RNA interference (RNAi) plants underline the importance of coordinated cell and organelle division for plant growth and morphogenesis.

Amphibian metamorphosis involves extensive, but selective, neuronal death and turnover, thus sharing many features with mammalian postnatal development. The antiapoptotic protein Bcl-X L plays an important role in postnatal mammalian neuronal survival. It is therefore of interest that accumulation of the mRNA encoding the Xenopus Bcl-X L homologue, termed xR11, increases abruptly in the nervous system, but not in other tissues, during metamorphosis in Xenopus tadpoles. This observation raises the intriguing possibility that xR11 selectively regulates neuronal survival during postembryonic development. To investigate this hypothesis, we overexpressed xR11 in vivo as a green fluorescent protein (GFP)-xR11 fusion protein by using somatic and germinal transgenesis. Somatic gene transfer showed that the fusion protein was effective in counteracting, in a dose-dependent manner, the proapoptotic effects of coexpressed Bax. When GFP-xR11 was expressed from the neuronal β-tubulin promoter by germinal transgenesis we observed neuronal specific expression that was maintained throughout metamorphosis and beyond, into juvenile and adult stages. Confocal microscopy showed GFP-xR11 to be exclusively localized in the mitochondria. Our findings show that GFP-xR11 significantly prolonged Rohon-Beard neuron survival up to the climax of metamorphosis, even in the regressing tadpole tail, whereas in controls these neurons disappeared in early metamorphosis. However, GFP-xR11 expression did not modify the fate of spinal cord motoneurons. The selective protection of Rohon-Beard neurons reveals cell-specific apoptotic pathways and offers approaches to further analyze programmed neuronal turnover during postembryonic development.

Certain features reminiscent of sexuality occur in the vegetative life cycle of some filamentous fungi such as Neurospora crassa. Hyphal fusions can occur between genetically different individuals, thereby endowing the new composite mycelium, a heterokaryon, with some of the advantages of heterozygosity usually associated with diploid organisms. In N. crassa, however, there are a number of incompatibility loci which prevent formation of heterokaryons unless the alleles at the incompatibility loci are identical in the two mycelia. The selection pressures that maintain incompatibility polymorphisms are not known. We suggest here that they are maintained because they prevent a kind of exploitation of heterokaryons by nuclei that are nonadaptive in homokaryons but that enjoy a proliferative advantage over other nuclei in heterokaryons. A mathematical model that abstracts the major features of the vegetative life cycle of Neurosopra crassa has been developed, and the action of selection in this model and various extensions of it is such as to maintain polymorphisms of vegetative incompatibility factors.

The armored scale insect, Aspidiotus simulans DeLotto, has been reported only for the central highlands of Kenya, in and near Nairobi, where it apparently replaces the closely allied, cosmopolitan species, A. destructor Sign., of the warmer coastal and inland areas. In most regards A. simulans is cytologically a typical armored scale. The females have eight chromosomes, and four bivalents appear at oogenesis. The males become haploid, with four chromosomes, by eliminating chromosome sets during early embryonic mitoses. The eliminated chromosomes are, however, considerably more active on the spindle, and much less contracted than is usual during this process in the armored scales. Although accurate counts were not possible, the number of symbionts in the embryos at very late cleavage was roughly the same for the four-chromosome (male) and the eight-chromosome (female) embryos. The females of collection No. 1 contained either all four-chromosome or all eight-chromosome embryos at postcleavage stages. In the...

1. Mature leaves of larkspur, for the aspects studied, bear a relationship to one another typical of growth sequences in single organs. 2. Slightly sigmoid curves were obtained when the characters (number of points per side, per middle, total, and area, length, and area/length) were plotted against physiological time intervals (leaf number). 3. Log-log plotting of some characters against others yields relative growth relationships expected from sigmoid developmental sequences. 4. The problem of the developmental factors underlying these relationships is briefly analyzed. 5. The number of points is in direct relationship to the area of the leaf, indicating that incision must be regarded as a segmentation of the area rather than a splitting in from the circumference. 6. The use of homologous organ series in problems of relative growth and the use of relative growth techniques in estimating degree of differentiation are discussed.

A member of the Orthezia praelonga species group has 16 chromosomes in the germ line of both the male and female, and shows no apparent differential behavior between the two haploid complements in the male. In not showing a chromosomal difference between the sexes, it is unlike any other bisexual species of coccid yet studied cytologically. In most respects, spermatogenesis appears to conform to the primitive coccid pattern. Various possible modes of sex determination, derivable from the basic coccid XO system, are considered in the discussion.

In unfixed tissues of the nucleolar wall of L. campestris, the nucleoli are occasionally bipartite or U-shaped, When bipartite, their appearance may be similar to that of heterochromatic elements visible in smear preparations.When treated with acetic acid, the atypical nucleoli react similarly to normal nucleoli and unlike the chromosomal substance. The atypical nucleoli are usually not stained by the custamary aceto-carmine technique but can be stained intensely following Rattenbury's formol modification.Atypical nucleoli are much more frequent in nuclei with two nucleoli than would be expected if their occurrence were not related to the binucleolate condition. The idea is offered that the atypical nucleoli are formed in part by a failure of the usual fusions of material collected by the organizers.A further observation of vacuoles in unfixed nucleoli is noted.

1. In aceto-carmine smears of Luzula campestris, the six bivalents at metaphase I appear to be associated in groups rather than spread out at random. The two most common arrangements are three groups of two bivalents and a one-two-three arrangement.2. In sectioned material, no such grouping is present. The secondary associations of the smears are therefore artifacts.3. In the sectioned material, the pollen-mother cells and the spindles at the equatorial plate are frequently asymmetric. The chromosomes vary in arrangement on the plate, to a large degree in accord with the asymmetry of the spindle. The three types of chromosome arrangement are the circular, the triangular, and the type with two rows.4. Comparisons, some of them quantitative, can be made between the arrangements of chromosomes in the sections and those of the smears. Factors which may aid in the production of the spurious secondary association are discussed.The writer is indebted to Dr. G. LEDYARD STEBBINS, Jr. for suggesting a cytological study of L. campestris, and to him and to Dr. ALOHA HANNAH for pertinent suggestions during the course of this work.

A survey of chromosome systems in the Eriococcidae shows that many of them are intermediates between two systems occurring elsewhere in the coccids. These two systems differ in the number of paternal chromosomes which remain heterochromatic during spermatogenesis and are then discarded. In the lecanoid system, all are discarded; in the Comstockiella system only one, the DH chromosome is discarded. Evolutionary steps from the lecanoid to the Comstockiella systems had previously been postulated with the intermediates having from n-1 down to 2 chromosomes remaining heterochromatic during spermatogenesis. The transition was believed to be reversible and the systems therefore inherently unstable. Most of the armored scales with Comstockiella systems have not shown intermediacy but one case has been recently reported. The eriococcids as a family are characteristically intermediate with frequently extreme variation from cyst to cyst in number of chromosomes remaining heterochromatic. Species with an unmodified Comstockiella system may occur but are in a minority. One unanticipated system is reported for two species, the “complete” Comstockiella system in which all the heterochromatic chromosomes are reversed at spermatogenesis; this system is also theoretically unstable. Indirect evidence for the great antiquity of the eriococcids is presented, and the question raised as to the maintenance of instability over several epochs. Comparative cytology may provide part of the answer to this question. The most common, and probably the basic diploid chromosome number of the eriococcids is 18, with a range from 12 to 28 and about 48; decreases in number are more frequent than increases.

1. Chromosome behavior at spermatogenesis has been described for one species each of three families of coccids not previously studied in this regard: Cerococcus quercusComstock of the Asterolecaniidae, Conchaspis lepageiHempel of the Conchaspidae, and Tachardiella sp. of the Lacciferidae. All three families belong to Balachowsky's “lecanoid” section of the coccid superfamily, and the first two have been suggested, by various systematists, as close allies of the armored scales (Diaspididae). 2. All three species showed lecanoid chromosome behavior, as originally described by the Sohraders, in that one haploid set of chromosomes is heterochromatic in the males and eliminated at spermatogenesis. The exceedingly clear cytology of C. quercus permitted a detailed study of the critical second division stages when the euchromatic and heterochromatic sets are separated. Although considerably more difficult cytologically, both the other species showed a typical lecanoid pattern of behavior; in addition, both showed an accessory structure, of unknown origin and function, which was present during the meiotic sequence. 3. The cytological and taxonomic implications of the results are considered, largely on a comparative basis, in the discussion. Special attention is given to the problem of the origin of the armored scales and also to Buchner's recent suggestion of the possible influence of symbionts on sex determination.

Chromosome imprinting is the process by which one of two genetically homologous chromosomes is predetermined to function differently from the other at a subsequent stage in development. In the coccid insects, imprinting occurs in the egg, at the time of fertilisation; it probably occurs at the same time and site in mammals, and possibly also in Sciara.

is not available.

THE mealy bug, P. citri, affords a unique opportunity for analysis of biochemical changes underlying chromosome behaviour. In the male embryos, the entire paternal set, that is, one half the total number of chromosomes, remains condensed in the non-dividing nuclei; this facultative behaviour is similar to that of typical heterochromatic elements such as sex chromosomes1,2. Experiments with tritiated thymidine show that, like heterochromatic elements in other organisms3,4, the paternal set in the males reproduces asynchronously from the maternal, or euchromatic, set5. As measured by dominant lethality2 and mutant genes6, the paternal set is genetically inert in the males, but some ‘residual’ influences can be demonstrated7,8.

THE chromosome constitution of marsupial, Potorous tridactylis, was described by Sharman and Barber1. The males have five pairs of autosomes and three sex chromosomes. The low number of chromosomes plus the fact that the same investigators reported somatic pairing made this species desirable for genetic studies in tissue cultures.

In the lecanoid chromosomal system, as exemplified by the mealy bug, Planococcus citri (Risso), one chromosome set becomes heterochromatic during embryogeny of the male and is maintained as such during development. At spermatogenesis, the first division is equational for both the euchromatic and heterochromatic chromosomes which are segregated from each other in the second; only the euchromatic derivatives form sperm. After paternal irradiation, the induced aberrations appear in the heterochromatic set of the male embryos while they occur in the euchromatic set after maternal treatment. After paternal irradiation of doses up to 30000 rep, dominant lethality is induced in daughters but not in sons. There is thus a clear picture of the failure of damage to the heterochromatic set to produce a detectable genetic effect. These results do not rule out a secondary genetic or physiological function of the heterochromatic set. No breakage-fusion-bridge cycles, which would complicate the results whether the heterochromatic set were inert or not, were observed in the cytological studies after low-dosage treatments. After maternal treatment, both sexes diminish with increasing dosage but the results are complicated by the effect of maternal age on the sequence in which the two sexes of offspring occur during oviposition (sexualmore » dichronism) as well as on total progeny. At high-dosage paternal treatments, 60, 90, and 120,000 rep, the number of sons is drastically reduced while the number of daughters is about 40% of the controls. (P.C.H.)« less

UNLIKE gene mutations the expression of which may be subject to considerable modification, chromosomal changes will remain the same whether they occur as sporadic aberrations or become incorporated in the karyotype of a species. If the complements of two closely related species differ, spontaneous chromosomal mutations paralleling the karyotypic differences should be demonstrable in the same group of organisms.

SUMMARY In the diaspine scale, Pseudaulacaspis pentagona, the males are haploid, with eight chromosomes, and the females are diploid, with 16. Meiosis is normal in the female and replaced by a simple mitosis in the male. Mating is required for the production of any offspring, either male or female. Egg production affords an example of combined sexual dimorphism and dichronism; the mothers first produce a series of eggs containing coral-colored female embryos and then, without interruption, lay a series containing pinkish-white male embryos. Cytological study of early embryogeny shows that a chromosome set is eliminated to produce the haploid condition of the male embryos. Chromosomes broken by X-rays were used as genetic markers to demonstrate that the eliminated set is of paternal origin. Symbionts, yeast-like in appearance, are present at the distal end of both male and female embryos. The polyploid sector of the embryo stems from the fusion of a cleavage nucleus with both polar bodies and is pentaploid with 40 chromosomes. As would be expected on a simple genetic basis, irradiation of fathers preferentially reduces the number of daughters.

Cytologically, Puto has proved more primitive than its taxonomic position would indicate. In the single previously described example (Hughes-Schrader, 1944), an uncomplicated inverted meiotic sequence was described for the males. The present example, P. albicans, showed a significantly more primitive inverted sequence. Unlike the other examples reported for aphids and coccids, the chromatids of the dyads neither dissociated nor reassociated during interkinesis. Instead, they remained closely associated and interconnected by an unresolved terminal chiasmate attachment. At first metaphase, the spindle attachments were localized to a restricted region of the poleward surface of the chromatids. Localization of attachment during meiosis and close association of chromatids during interkinesis are both suggestive of similar but not identical conditions expected in ancestors with an uninverted meiotic sequence. A second species proved intermediate between P. albicans and that described by Hughes-Schrader in which a complete cycle of dissociation and reassociation occurred during interkinesis. In P. albicans the pachytene bivalents showed no structures suggestive of centric localizations. At their greatest condensation at first metaphase, the chromatids were clearly subdivided into half chromatids. Limited observations were made on chromosomes of other species of Puto and of Phenacoleachia zealandica, and also on spermiogenesis and mycetocyte formation in Puto. The discussion is devoted to considerations of chromatid subdivision, holokinetic chromosomes, and meiotic inversion and some evolutionary implications are mentioned.

Conventional types of cytogenetic studies with the mealybug, Planococcus citri (Risso), are possible with the use of genetic markers and meiotic analysis in the female. The loci of an eye-color mutant, salmon, and a wing-shape mutant, banjo, are linked with about 22 per cent recombination. These markers have been used in the identification and maintenance of lethals and rearrangements. All the cytologically identifiable rearrangements have proved to be reciprocal translocations, some symmetric, others, grossly asymmetric or otherwise complicated. No simple breakage products have been recovered. On the basis of their effects on crossing over, some of the lethals are believed to be associated with small rearrangements. The bivalents normally have one chiasma; only 1.2 per cent have two. Interference is decidedly decreased in chiasma formation in translocation heterozygotes, and in genetic recombination with suspected small rearrangements associated with lethals; it is also decreased, but less markedly, in genetic recombination with lethals in translocations. These various results are discussed in relationship to the holokinetic nature of the coccid chromosome, and natural increases in coccid chromosome number, as well as in regard to the effect of rearrangements on interference.

Monograph of cimicidae (Hemiptera-Homoptera). Robert L. Usinger. Published by the Entomological Society of America, College Park, Maryland, 1966. Pp. xi+585. $10.00.