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.     

Inter-embryo competition in the progeny of autotriploid Aloineae (Liliaceae)

In reciprocal crosses between diploid and triploid Aloineae the progeny are largely diploid or diploid plus one or two chromosomes, but in reciprocal crosses between triploids and tetraploids they are tetraploid or nearly so. Thus the triploids contribute circa haploid gametes to the progeny when crossed with diploids but circa diploid gametes when crossed with tetraploids. These results are compared with those of a number of earlier workers. It is concluded that the bias in the frequency of progeny types towards diploidy or tetraploidy, depending on the ploidy level of the plant which is crossed with the triploid, is caused by inter-embryo competition. Those embryos with an endosperm/embryo factor of 1.5, the value found in normal diploid/diploid crosses having triploid endosperms, are selected in preference to those with factors higher or lower than 1.5.Inter-gamete competition also occurs among the euploid and aneuploid gametes produced by the triploids. This is more pronounced on the male side, because the degree of survival of aneuploid pollen from the triploids into the next generation is much lower than that of aneuploid egg nuclei.Non-reduction in the triploids gives rise to occasional pentaploid progeny in crosses with tetraploids, but it is more probable that in diploid/triploid crosses tetraploid progeny are the products of non-reduction in the diploid.     

Cytogenetic studies of Poecilia (Pisces)

Natural populations of triploid females resembling the gynogenetic teleost, Poecilia formosa (Girard), occur in northeastern Mexico where they intermingle with diploid populations of this species and the members of congeneric bisexual species such as P. mexicana or P. latipinna. Mitotic configurations from gill epithelial cells show 46 chromosomes for the diploid fishes, but 69 chromosomes for members of the triploid clones associated with P. formosa. Triploid females have erythrocytes that are significantly larger than those from diploid specimens and also show a roughly 50% elevation in the average DNA content of their somatic nuclei. Similar analyses of two functionally incompetent males of P. formosa, of a number of bisexual F₁ and F₂ hybrid offpsring from P. latipinna x P. mexicana, and of females from several other poeciliid species consistently show only diploid DNA levels and somatic chromosome complements where 22N=46. Demonstration of cytogenetic criteria by which females from triploid clones may be clearly distinguished from sympatric diploid specimens of P. formosa or P. mexicana leaves unresolved, for the present, problems of an appropriate systematic designation for natural populations of triploid gynogenetic fishes. The role of sympatric speciation in the evolution of poeciliid genomes is discussed in terms of alternative mechanisms to account for the persistence in nature of a vertebrate triploid of hybrid origin.This work was supported by grants from the National Science Foundation (GB 7393) and from the U.S. Public Health Service (GM 14644).Recipient of a Research Career Development Award from the U.S. Public Health Service (1 K3 GM 3455).     

Molecular genetics of growth and development in Populus. I. Triploidy in hybrid poplars

Summary While constructing a genetic linkage map of a hybrid poplar genome (Populus trichocarpa x P. deltoides), we identified several restriction fragment length polymorphismus (RFLPs) for which the parental trees are heterozygous. Although 8 of the 11 F₁ hybrid offspring inherited, as expected, single RFLP alleles from each parent, 3 F₁ trees in the mapping pedigree inherited both maternal alleles along with a single paternal allele at some loci. Aneuploidy or polyploidy in these 3 F₁ trees due to partial or complete nondisj unction during female gametogenesis is the simplest explanation for this finding. Of the 3 f₁ offspring with supernumerary RFLP alleles 2 have triploid nuclear DNA contents as measured by fluorescence flow cytometry; the 3rd F₁ with supernumerary alleles has a sub-triploid nuclear DNA content and is probably aneuploid. Among the tri/aneuploid hybrids, leaf quantitative traits either are skewed toward those values characteristic of the P. trichocarpa female parent (adaxial stomate density, petiole length: blade length ratio; abaxial color) or show transgressive variation (epidermal cell size). Abaxial leaf color was used to screen a large population of P. trichocarpa x P. deltoides hybrids for further evidence of tri/aneuploidy. In each case where a white abaxial leaf surface was observed and the nuclear DNA content measured, the hybrid proved to be tri/aneuploid. All sexually mature female triploids examined were sterile, although the inflorescences completed their development in the absence of embryo formation. The (probably) aneuploid F₁ hybrid is a fertile female. Of 15 female P. trichocarpa parents used in crosses to P. deltoides, 10 produced one or more tri/aneuploid hybrid offspring. In an intraspecific cross using a P. trichocarpa female that had produced triploid hybrids with five different P. deltoides males, no tri/aneuploid offpsring were found.     

Discovery of a diploid population of theHemidactylus garnotii-vietnamensis complex (Reptilia: Gekkonidae)

A diploid member of the parthenogenetic gekkonid species complexHemidactylus garnotii-vietnamensis was discovered for the first time from Thailand. This gecko, seemingly unisexual and parthenogenetic, possesses 2n=2x=38 chromosomes, showing distinct heteromorphisms. The absence of bisexual congeneric species with a combination of karyomorphs to produce this karyotype indicates the occurrence of chromosomal rearrangements after the initial estabilishment of a diploid clonal lineage of hybrid origin. Results of karyotypic comparisons of the present sample and the three known triploid species belonging to theH. garnotii-vietnamensis complex suggest that a triploid karyomorph similar to that ofH. vietnamensis has first emerged through an insemination of the diploid parthenogen's egg by the sperm from a bisexual species having 44 chromosomes (all telocentric), and that the karyomorph subsequently experienced some minor chromosomal aberrations to produce the karyomorphs ofH. vietnamensis andH. garnotii. The origin of theH. stejnegeri karyotype still remains an open question for future studies.     

STABLE DIPLOIDS FROM INTRAGROUP ZYGOSPORES OF CLOSTERIUM EHRENBERGII MENEGH. (CONJUGATOPHYCEAE)1

Two pairs of stable diploid clones were obtained as aberrant forms among F₁ progeny of an intragroup (intraspecific) cross between R-11-4 (mating type +) and M-16-4b (mating type -) of Group A of Closterium ehrenbergii Menegh. Each pair was derived from the two germination products of a single zygospore, and both clones were mating type minus. The cell size range of these four diploid minus clones was considerably above that of normal (haploid) Group A clones. Chromosome counts at the second meiotic metaphase indicated that these clones were diploid with approximately 200 chromosomes, which was double the number for normal Group A clones. Diploid minus clones conjugated normally with any haploid Group A plus clones, and yielded many triploid zygospores. Triploid zygospores germinated normally as did intragroup diploid zygospores. In metaphase I preparations, only bivalents were observed except on a few occasions where some uni- and multivalents were also detected. Viability of F₁ progeny from triploid zygospores (55–74%) was somewhat lower than from diploid zygospores of Japanese Group A populations (65–90%), but higher than intergroup (interspecific) hybrid zygospores from Groups A, B and H (0–12%). In addition to lower viability, some F₁ progeny from triploid zygospores exhibited slow vegetative growth. Almost all pairs of F₁ clones from single triploid zygospores were of opposite mating type, similar to normal diploid zygospores of the intragroup cross. Morphological variability of F₁ progeny of triploid zygospores was great. The apparently normal meiosis of triploid zygospores and the high viability of F₁ progeny suggested that the genome of Group A contains several sets of chromosome complements with mechanisms by which bivalents are regularly formed in the first meiotic division.     

Genetic polymorphism and evolution in parthenogenetic animals. I. Polyploid curculionidae

The genetic variability at enzyme loci in different triploid and tetraploid parthenogenetic weevil populations has been elucidated by starch gel electrophoresis. The overall genotype of individual weevils belonging to different populations has been determined for over 25 loci. The results are compared with those obtained for diploid bisexual races of either the same or closely related species. The variation within a parthenogenetic population differs from that in diploid, sexually reproducing populations, i.e. the allele frequencies are not in a Hardy-Weinberg equilibrium. The results indicate that apomictic parthenogenetic populations can differentiate genetically. The genotypes within a population resemble each other more than genotypes belonging to different populations. It is evident that evolution still continues—even if slowed down—in parthenogenetic weevils. A comparison between the allele relationships in geographically isolated polyploid parthenogenetic populations and related diploid bisexual forms does not support the hypothetical hybrid origin of parthenogenesis and polyploidy in weevils. Parthenogenesis within a parthenogenetic weevil species is evidently monophyletic.     

Genetic Polymorphism and Evolution in Parthenogenetic Animals. II. Diploid and Polyploid SOLENOBIA TRIQUETRELLA (Lepidoptera: Psychidae)

Genic polymorphism at sixteen enzyme loci of four different chromosomal races of Solenobia triquetrella (bisexual, two diploid parthenogenetic races and tetraploid parthenogenetic) has been studied by starch gel electrophoresis. Isolated small diploid bisexual populations have rather uniform allele frequencies at all loci which we have studied. Diploid and tetraploid parthenogenetic individuals of this species are in general as heterozygous as bisexual ones. All parthenogenetic local populations are different from each other in the Alps. These parthenogenetic genotypes cannot be derived from a common ancestor through single mutations but rather bear evidence for a polyphyletic origin of parthenogenesis in Solenobia triquetrella. In the marginal distribution areas of the species in northern Europe single genotypes are spread over far larger areas than in the mountain regions of central Europe. This may be due to the old origin of parthenogenesis and polyploidy in northern Europe. No new parthenogenetic and polyploid strains have lately arisen in the regions outside of the Alps.     

Sex determination in bees. I. Balance between femaleness and maleness genes inBombus atratus franklin (Hymenoptera,Apidae)

Mating between a diploid male and a diploid female ofBombus atratus produced fertile triploid F₁ females. The F₂ descendents of these virgin females were composed of haploid males (10), diploid males (4), aneuploid males (3) and intersexes (2). These data indicate that sex is produced by a balance between male determining and female determining genes: they, also, suggest that the number of sex genes are not large.     

Heterochromatin variation in Cryptobothrus chrysophorus

Synthetic F₁ hybrids between individuals taken from selected populations of the northern (Glenn Innés, Bolivia Hill) and southern (Forbes Creek, Mount Aggie) chromosome races of the grasshopper Cryptobothrus chrysophorus confirm that these two races are indeed distinguished by fixed differences in heterochromatin content. These differences affect five of the six medium sized members of the complement (M 4, 5, 6, 8 and 9). Added to this there is a marked change in the character of the remaining medium pair (M 7) which functions as the megameric in this species. — Meiosis in F₁ males is characterised by the presence of univalency in from 22–32% of the meiocytes. This appears to be genotypic in causation and may involve from one to three chromosome pairs in any one cell. Laggards produced from such univalents lead to a failure of anaphase separation at either first or second division or at both of them. Consequently from 17–36% of the sperm produced are giant (diploid or tetraploid) in size. Added to this the medium members commonly form end to end associations involving the heterochromatic (northern race) and euchromatic (southern race) terminii. This is a modified form of the heterochromatic end associations which occur between homologous medium pairs in northern populations. The two small pairs (S10 and 11) show comparable behaviour in both races when they carry terminal heterochromatic supernumerary segments. Such associations persist despite the fact that they are non-chiasmate in character. Moreover, despite the asynapsis found in F₁ males they produce F₂s when allowed to intersib mate, although many of the embryos fail to develop, presumably as a result of genotypic imbalance, or else are aneuploid in constitution. In the F₂s that do survive to maturity the chromosome differences which distinguish the northern and southern forms are recombined to give a wide variety of karyotypes in which the observed pairing is much improved and from which F₃s were subsequently obtained.     

Production of Polyploids and Unreduced Gametes in Lilium auratum × L. henryi Hybrid

Intergenomic F₁ hybrids between L. auratum x L. henryi and their BC₁ progeny were investigated through genomic in situ hybridization technique (GISH) to determine their potential value in lily breeding. We confirmed that F₁ intergenomic hybrids possessed a set of chromosomes (x=12) from both parents and that flowers of the F₁ auratum × henryi hybrid showed an intermediate morphological phenotype. Pollen size, viability and germination ability were measured through microscopic observations. F₁ intergenomic hybrids produced a relevant frequency of 2n-gametes, which were successfully used to perform crosses with Oriental hybrids, resulting in the triploid Oriental Auratum Henryi (OAuH) hybrid. Twenty BC₁ plants were generated by crossing between four different Oriental hybrid cultivars and F₁ AuH hybrids using an in vitro embryo rescue technique, after which the genome constitution and chromosome composition were analyzed by GISH. All plants were triploid, showing 12 from female parents (diploid Oriental hybrid) and 24 from male parents (diploid F₁ AuH hybrid). Overall, 16 out of 20 BC₁ progeny possessed recombinant chromosomes with 1-5 crossover sites per plant. Cytological analysis of 20 BC₁ plants by GISH verified that the occurrence of 2n pollen formation in all F₁ AuH hybrids was derived from the FDR (first division restitution) mechanism, in which the genome composition of all BC₁ plants possess 12 Oriental + 12 L. auratum + 12 L. henryi chromosomes. Allotriploids derived from the AuH hybrid were used as female for crossing with the diploid Oriental hybrid cultivar ''Sorbonne'' and considerable numbers of plants (0-6.5 plants per ovary) were only obtained when female OAuH (BC₁) triploids were used. Taken together, the results of this study indicate that production and analysis of F₁ AuH hybrids and their progeny through sexual polyploidization can be useful for efficient creation of important horticultural traits.     

Transfer of bacterial blight and blast resistance from the tetraploid wild rice Oryza minuta to cultivated rice,Oryza sativa

Summary Oryza minuta J. S. Presl ex C. B. Presl is a tetraploid wild rice with resistance to several insects and diseases, including blast (caused by Pyricularia grisea) and bacterial blight (caused by Xanthomonas oryzae pv. oryzae). To transfer resistance from the wild species into the genome of cultivated rice (Oryza sativa L.), backcross progeny (BC₁, BC₂, and BC₃) were produced from interspecific hybrids of O. sativa cv IR31917-45-3-2 (2n=24, AA genome) and O. minuta Acc. 101141 (2n=48, BBCC genomes) by backcrossing to the O. sativa parent followed by embryo rescue. The chromosome numbers ranged from 44 to 47 in the BC₁ progeny and from 24 to 37 in the BC₂ progeny. All F₁ hybrids were resistant to both blast and bacterial blight. One BC₁ plant was moderately susceptible to blast while the rest were resistant. Thirteen of the 16 BC₂ progeny tested were resistant to blast; 1 blast-resistant BC₂, plant 75-1, had 24 chromosomes. A 3 resistant: 1 susceptible segregation ratio, consistent with the action of a major, dominant gene, was observed in the BC₂F₂ and BC₂F₃ generations. Five of the BC₁ plants tested were resistant to bacterial blight. Ten of the 21 BC₂ progeny tested were resistant to Philippine races 2, 3, and 6 of the bacterial blight pathogen. One resistant BC₂, plant 78-1, had 24 chromosomes. The segregation of reactions of the BC₂F₂, BC₂F₃, and BC₂F₄ progenies of plant 78-1 suggested that the same or closely linked gene(s) conferred resistance to races 2, 3, 5, and 6 of the bacterial blight pathogen from the Philippines.     

The inheritance of radiation induced semi-sterility in Rhodnius prolixus

Eggs from crosses of 40 adult male R. prolixus irradiated with 6K rad -rays with normal females had a mean fertility of 23.9%, only 2 crosses being completely sterile. The 86 F₁ progeny of both sexes, when outcrossed with normal mates, had a mean egg fertility of 12.6%, and 43 of these matings were completely sterile. Twenty-eight F₂ bugs reared from F₁ x normal crosses were mated with normal partners and had a mean fertility of 44.6%, 6 of them being fully fertile, a reversal towards normal fertility. Cytogenetic examination of F₁ F₂ and F₃ males showed that these changes in fertility correlated well with the degree of chromosomal abnormality found. The very high recovery rate of translocations in F₁ generation males can be related to the holocentric chromosomes of these bugs which precludes the formation of dicentric chromosomes which are inviable in monocentric species. In F₁ and F₂ males the majority of translocations were associated as chains of III+I or as chains of IV. Only one bug was found with a ring of IV chromosome association and it is suggested that chromosome morphology, combined with a low chiasma frequency, favours chain association. Most chain multivalents showed linear orientation which may lead to duplication deficiencies and zygotic death. However parallel, indifferent and the more stable convergent modes of chain orientation were also all observed indicating that survival of some translocations in this species may be possible. The survival to the F₂ generation of chromosomal fragments confirmed the holocentric nature of triatomine chromosomes. It is suggested that semi-sterile males would prove more effective than releases of completely sterile males for reducing wild populations of R. prolixus, because of the delayed effects of sterilizing radiation consequent upon the holocentric structure of triatomine chromosomes.     

Studies on the cytotypes of Atrichum undulatum (Hedw.) P. Beauv. II. Chromosome length and relative DNA content

Abstract

Evidence is presented showing that the chromosomes of diploid and triploid races of Atrichum undulatum are significantly shorter than those of haploid plants. Relative DNA contents of the three cytotypes have been estimated and they differ significantly from an expected 1:2:3 ratio in haploid, diploid and triploid races.     

The genetics of adult-plant blackleg (Leptosphaeria maculans) resistance from Brassica juncea in B. napus

The genetic control of adult-plant blackleg (Leptosphaeria maculans) resistance in a Brassica napus line (579NO48-109-DG-1589), designated R13 possessing Brassica juncea-like resistance (JR), was elucidated by the analysis of segregation ratios in F₂ and F₃ populations from a cross between R13 and the highly blackleg-susceptible B. napus cultivar Tower. The F₂ segregration ratios were bimodal, demonstrating that blackleg resistance in R13 was controlled by major genes. Analysis of the segregation ratios for 13 F₃ families indicated that blackleg resistance in these families was controlled by three nuclear genes, which exhibited a complex interaction. Randomly sampled plants of F₃ progeny all had the normal diploid somatic chromosome number for B. napus. The similarities between the action of the three genes found in this study with those controlling blackleg resistance in B. juncea is discussed.     

Evidence of Different Ploidy Eggs Produced by Diploid F2 Hybrids of Carassius auratus (♀) × Cyprinus carpio (♂)

Based on the presence of three types of eggs with different diameters 0.13, 0.17 and 0.2 cm, we made two crosses: F₂ (♀) × diploid red crucian carp (♂), and F₂ (♀) × F₁₀ tetraploid (♂). The ploidy levels of the progeny of the two crosses were examined by chromosome counting and DNA content measurement by flow cytometer. In the offspring of the former cross, tetraploids, trip-loids, and diploid were obtained. In the progeny of the latter cross, tetraploids and triploids were observed. The production of the different ploidy level fish in the progeny of the two crosses provided a further evidence that F₂ might generate triploid, diploid and haploid eggs. The presence of the male tetraploid found in F₂ (♀) × diploid red crucian carp (♂) suggested that the genotype of XXXY probably existed in the tetraploid progeny. The gonadal structures of the tetraploids and triploids indicated that both female and male tetraploids were fertile and the triploids were sterile. We concluded that the formations of different ploidy level eggs from F₂ were contributed by endoreduplication and fusion of germ cells.     

Population cytogenetics of the genus Caledia (Orthoptera: Acridinae)

The acridine grasshopper, Caledia captiva exists as two chromosomal races in south-east Queensland. One of these, the Moreton race inhabits the coastal region to the east of the Great Dividing Range. All chromosomes of the complement (2n=11II+XO/XX) have been involved in centromeric rearrangement, which transforms the acro- and telocentric chromosomes into submeta- and metacentric elements. The second, or Torresian race is widely distributed through southern Papua, Arnhem Land, Cape York Peninsula and down the east coast of Australia as far south as Brisbane. This race, which is characterised by a completely acro- and telocentric chromosome complement, approaches the Moreton race in south-east Queensland where the two races are separated by less than 1 km, along a front of at least 150 km. Evidence is presented to show that chromosome introgression is occurring across the contact zone and this takes place in one direction only, namely the Torresian chromosomes are infiltrating into the Moreton race but not reciprocally. Furthermore, the introgression of chromosomes across the zone is limited to certain members of the Torresian complement and even then these successful chromosomes show highly variable degrees of penetrance into the Moreton race. It is proposed that a tension zone exists between these two races which is maintained by the interaction of (a) ecological tolerance differences on either side of the zone and (b) by partial competitive exclusion due to the interracial differences in phenology. This case of parapatric association with limited hybridisation is unique in its clarity due to the marked differences in the appearance of the chromosome complements of these races which permits direct assessment of the behaviour of most members of the genome in hybrids and their derivatives.     

The Organization of Genetic Diversity in the Parthenogenetic Lizard CNEMIDOPHORUS TESSELATUS

The parthenogenetic lizard species Cnemidophorus tesselatus is composed of diploid populations formed by hybridization of the bisexual species C. tigris and C. septemvittatus, and of triploid populations derived from a cross between diploid tesselatus and a third bisexual species, C. sexlineatus. An analysis of allozymic variation in proteins encoded by 21 loci revealed that, primarily because of hybrid origin, individual heterozygosity in tesselatus is much higher (0.560 in diploids and 0.714 in triploids) than in the parental bisexual species (mean, 0.059). All triploid individuals apparently represent a single clone, but 12 diploid clones were identified on the basis of genotypic diversity occurring at six loci. From one to four clones were recorded in each population sampled. Three possible sources of clonal diversity in the diploid parthenogens were identified: mutation at three loci has produced three clones, each confined to a single locality; genotypic diversity at two loci apparently caused by multiple hybridization of the bisexual species accounts for four clones; and the remaining five clones apparently have arisen through recombination at three loci. The relatively limited clonal diversity of tesselatus suggests a recent origin. The evolutionary potential of tesselatus and of parthenogenetic forms in general may be less severely limited than has generally been supposed.     

Population cytogenetics of the genus Caledia (Orthoptera: Acridinae)

The genus Caledia contains two species. C. species nova 1 is restricted to the Oriomo Plateau of S.W. Papua and has a complement of twelve telocentric chromosomes. The second species C. captiva has a much wider distribution pattern—from S.W. Papua in the North, down the entire Eastern seaboard of Australia to Southern Victoria. It is also found in the Northern Territory. Although the chromosome number is the same as C. species nova 1, four major and distinct chromosomal races can be distinguished in C. captiva. — The basic ancestral race is found in Tropical North Queensland at the base of the Cape York Peninsula. All twelve chromosomes are telocentric and the karyotypic organization is similar to that found in C. species nova 1 and in other Acridines. A second, general purpose karyotypic race has a wide distribution between S.W. Papua, Arnhem Land and the East Australian coast as far South as Brisbane. It is considered a derivative form of the ancestral type and is fixed for small pericentric inversions on seven pairs of chromosomes. In the South-Eastern Queensland region there exists a further race which carries large pericentric inversions on all the autosomes and the X chromosome. The situation here is confounded since the basic chromosomes can be represented as either acro or telocentrics. Various levels of polymorphism for the inversions exist between different chromosomes in different populations indicating considerable differentiation within this zone. This race is almost completely surrounded by the general purpose karyotype where the races are contiguous in certain parts of the range. — The South-Eastern corner of Australia is characterised by a chromosome race quite different from those found further North. Here a complex pericentric inversion system exists involving a series of seven small inversions and larger inversions on chromosomes 1, 2, 4 and 10. Chromosomes 2 and 4, in particular, are highly polymorphic. — The presence and persistence of these 4 chromosomal races can be accounted for in terms of the known climatic changes which have occurred in this region in the recent past.     

Cytogenetic study of Artemia from Urmiah, Maharloo and Incheborun Lakes

Three populations of Artemia from Urmiah (West-Azarbaijan Province), Maharloo (Fars Province) and Incheborun (Golestan Province) Lakes in Iran were cytogenetically analyzed. In order to study the reproduction mode in Artemia populations, cyst samples reared for three successive generations (F₁--F₃). The results obtained from rearing until maturation indicated that, there are two subpopulations of Artemia in Urmiah lake (bisexual and parthenogenetic). These two subpopulations have the same diploid chromosome number (2 n =42). The results obtained from rearing and cytogenetic analysis of Artemia from Maharloo and Incheborun Lakes showed that these two populations are parthenogenetic and their chromosome number is also 2 n=42. So far, no cytogenetic differences in ploidy level have been detected among the populations investigated in this study. Very small chromosomes with metacentric, submetacentric and telocentric morphology were observed in mitotic prometaphase cells, implying that chromosomes of Artemia may be monocentric.