Oogenesis and Embryology of Two Plant-parasitic Nematodes,Pratylenchus penetrans and P. zeae

The process of oogenesis was studied in the bisexual species, Pratylenchus penetrans, and the monosexual species, P. zeae. The nucleus of the oocyte of P. penetrans underwent two divisions after sperm penetration. The chromosome number of P. penetrans observed at metaphase of the first maturation division was 2n = 10 and the reduced chromosome number observed at anaphase of the first maturation division was n = 5. Two polar bodies were found within the egg, indicating that this species reproduces by amphimixis. The nucleus o f the oocyte of P. zeae underwent one mitotic division and the chromosome number was 2n = 26. The presence of only 1 polar body indicates that this species reproduces by mitotic parthenogenesis. The development of the embryo was similar in P. penetrans and P. zeae. Unsegmented eggs were usually deposited by females. Following the 9-celled stage, the number of cells increased rapidly until a blastula was formed. Cell differentiation immediately followed, as evidenced by the formation of darker and larger inner endodermal cells and smaller ectodermal cells. Six to 7 days after egg deposition, the first stage larva was coiled three to f o u r times within the egg shell. During the first molt, the styler apex was formed first, then the larva moved frequently and vigorously and the styler was repeatedly thrust into the egg shell. Finally, the shell was broken and the second stage larva emerged. It took 10 days from the unsegmented egg to hatching at 23 C.     

Oogenesis and the Chromosomes of the Parthenogenic Root-knot Nematode Meloidogyne incognita

220 populations of Meloidogyne incognita and related forms from 46 countries reproduced by mitotic parthenogenesis (apomixis). Determination of somatic chromosome numbers from oogonia and oocytes revealed the existence of a predominant, possibly triploid race A with 3n = 40 to 46 and a rare, diploid race B with 2n = 32 to 36 chromosomes. There is no correlation between cytological races and the four recognized host races of this species. The characteristic behavior of prophase I chromosomes of maturing oocytes, which results in a prolonged prophase stage, is a unifying feature of all forms of M. incognita and supports monophyletic evolution, distinct from that of other Meloidogyne species. Extensive chromosomal polymorphism detected among populations can be helpful in elucidating the cytological pathway of evolution of the species.     

Oogenesis and the Chromosomes of the Cystoid Nematode,Meloidodera floridensis

Three Meloidodera floridensis populations of different geographic or host origin all reproduced by mitotic parthenogenesis. One of them from pine had a somatic chromosome number of 26, whereas, another population from pine and one from azalea had 2n = 27 chromosomes. All are considered to be triploid forms derived from an amphimictic ancestor with n = 9 chromosomes, the basic number in the closely related genus Heterodera. Evidence is presented which suggests that during division the chromosomes of the germ-line cells of the developing embryo behave differently than the chromosomes of all other blastomeres.     

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.     

Meloidogyne platani n. sp. (Meloidogynidae), a Root-knot Nematode Parasitizing American Sycamore

Meloidogyne platani n. sp. is described and illustrated from specimens obtained from roots of American sycamore, Platanus occidentalis, in Virginia. This new species shows certain similarities with M. arenaria but differs from it by a number of distinctive characters. The perineal pattern of females is rounded with fine, wavy to zig-zag striae and raised, convoluted striae in the inner lateral line regions. The stylet of females is 16.5 μm long with large, rounded stylet knobs set off from the shaft. Males have a low head cap and smooth head region. The styler length is 22.0 μm, and the stylet knobs are rounded and set off from the shaft. Mean second-stage juvenile length is 443.0 μm, and stylet length is 12.2 μm. The head region of juveniles is not annulated, and the tail has a definite terminus. This nematode causes severe galling and reproduces well on sycamore. Other good hosts include white ash and tobacco cv. NC 95. M. platani n. sp. reproduces by mitotic parthenogenesis and has a somatic chromosome number of approximately 45 (2n).     

Gametogenesis in Amphimictic and Parthenogenetic Populations of Aphelenchus avenae

The fetnale reproductive system of Aphelenchus avenae, studied in orcein-stained material, showed a peculiar structural pattern not yet reported in other nematodes. Chromosome morphology and behavior during gametogenesis could be studied in more detail than in other tylenchid or aphelenchid species investigated to date. In a bisexual population from Australia, gametogenesis was by normal meiosis and reproduction by amphimixis. The haploid chromosome number was n=8 in both males and females, and no sex chromosomes were detected. Three monosexual populations from Australia, California, and North Carolina underwent oogenesis by meiosis but reproduced hy parthenogenesis. The haploid chromosome number was n=8 in the Australia and the North Carolina populations, but n=9 in the California population. Spermatogenesis in temperature-induced males of the California population was by normal meiosis, and sperm had n=9 chromosomes. Most chronmsomes consisted of a central euchromatic section and two characteristic heterochromatic ends. No centromere was observed in any chronmsome. The relationship hetween the California population with n=9 and all the other populations with n=8 chromosomes is not well understood.     

Oogenesis and Reproduction of the Birch Cyst Nematode,Heterodera betulae

Cytological study revealed that maturation of oocytes of Heterodera betulae is by regular meiosis and reproduction is by parthenogenesis. Restoration of the somatic chromosome number occurs after telophase II and before egg pronucleus formation, in the absence of a mitotic apparatus through a type of endomitotic division. The haploid chromosome number is 12 (2n = 24) in 95% of the female nematodes studied and 13 in the remaining 5%. The phylogenetic relationship of H. betulae with most other Heterodera species having n = 9 is not clear.     

Morphological Comparison of Meloidogyne Males by Scanning Electron Microscopy

Males of five populations of Meloidogyne hapla were compared by scanning electron microscopy (SEM). Three populations of race A had haploid chromosome numbers of 15, 16, and 17 and reproduced by facultative parthenogenesis. Race B consisted of two mitotically parthenogenetic populations with somatic chromosome numbers of 45 and 48. Males of one population each of M. arenaria, M. incognita, and M. javanica were also examined to delineate species differences. The populations of M. arenaria, M. incognita, and M. javanica had 54, 41-43, and 44 chromosomes, respectively, and reproduction was by mitotic parthenogenesis. Observations were made on head structures, lateral field, excretory pore, and tail. The expression of labial and cephalic sensilla, shape and proportion of labial disc and lips, and markings on the head region were distinctly different for each species. The head morphology of the two cytological races of M. hapla was dissimilar. Populations of race A were different from each other and showed intrapopulation variation. Populations of race B were morphologically similar and stable in head morphology. The structure of the lateral field, excretory pore, and tail was of little value in distinguishing species or populations because of inter- and intrapopulation variation. The results are discussed in relation to earlier SEM observations of second-stage juveniles of the same populations.     

Gametogenesis and the Chromosomes of Two Root-knot Nematodes,Meloidogyne graminicola and M. naasi

Oogenesis and spermatogenesis were studied in populations of M. graminicola and M. naasi from which the species were originally described. Maturation of oocytes and spermatocytes in both species was by normal meiosis. The haploid chromosome number determined during the first and second maturation divisions was n = 18 with no variation. The somatic chromosome number determined in early cleavage divisions and, to a limited extent, in oogonial divisions was 2n = 36. Reproduction was regularly by meiotic parthenogenesis in both species. Re-establishment of the somatic chromosome number in mature oocytes, apparently took place through fusion of the second polar nucleus with the egg pronucleus. Occasional reproduction by cross-fertilization was demonstrated in M. graminicola and it is suspected in M. naasi in cultures with abundant males. Phylogenetic relationships in the family Heteroderidae are discussed in the light of the new cytological information. The peculiar behavior of nucleoli persisting during metaphase, anaphase and telophase of cleavage divisions is reported.     

Meloidogyne hispanica n. sp. (Nematoda: Meloidogynidae), the 'Seville Root-Knot Nematode'

Meloidogyne hispanica n. sp. is described and illustrated from specimens obtained from peach rootstock, Prunus persica silvestris Batsch, from the Seville district of Spain. The perineal pattern of the female is oval shaped to rectangular with low dorsal arch and often widely spaced lateral lines with fringe-like striae. The stylet, 14.1 μm long, has broad, distinctly set off knobs. Males have a high, rounded head cap that slopes posteriorly. Labial disc and medial lips are fused to form elongate lip structures. The robust styler, 23.5 μm long, has large, rounded knobs that are slightly set off from the shaft. Mean second-stage juveniles length is 392.6 μm. The truncate head region is generally not annulated. The distinctly rounded and raised labial disc and the crescent-shaped medial lips form dumbbell-shaped lip structures. The stylet, 11.1 μm long, has rounded, posteriorly sloping knobs. The slender tail, 46.4 μm long, has large irregular-sized annules in the posterior region and ends in a bluntly rounded tip. Tomato was a good host; tobacco, pepper, and watermelon were poor hosts; cotton and peanut were nonhosts. Meloidogyne hispanica n. sp. reproduces by mitotic parthenogenesis and has a somatic chromosome number of 2n = 33-36. The esterase pattern is unique among Meloidogyne species.     

Meloidogyne morocciensis n. sp. (Meloidogyninae), a Root-knot Nematode from Morocco

Meloidogyne morocciensis n. sp. is described from specimens parasitic on peach rootstock from Morocco. This species exhibits a combination of morphological characters similar to M. arenaria, M. incognita, and M. javanica. The perineal pattern of females is oval to squarish with a moderately high to high dorsal arch, and widely spaced, smooth striae; lateral lines are absent. The stylet, 16.5 μm long, has transversely ovoid, set-off knobs. Males have a set-off, annulated head region. The large, rounded labial disc is distinctly demarcated from the crescent-shaped medial lips; lateral lips are absent. The robust stylet, 24.6 μm long, has large, rounded knobs that taper slightly posteriorly. Mean second-stage juvenile (J2) length is 401 μm. The set-offhead region has incomplete annulations; the lip structures are dumbbell shaped. The stylet, 12.3 μm long, has rounded knobs that slope posteriorly. The J2 tail, 52.6 μm long, has irregularly sized annules in the posterior region and ends in a bluntly rounded tip. Tomato, tobacco, pepper, and watermelon are good hosts; cotton and peanut are not hosts. Meloidogyne morocciensis n. sp. reproduces by mitotic parthenogenesis and has a somatic chromosome number of 47-49. Its esterase phenotype is identical with the three-banded phenotype (A3) of M. arenaria.     

Meloidogyne mayaguensis n. sp. (Meloidogynidae), a Root-knot Nematode from Puerto Rico

Meloidogyne mayaguensis n. sp. is described and illustrated from specimens obtained from galled roots of eggplant, Solanum melongena L., from Puerto Rico. The perineal pattern of females is round to ovoid with fine, widely spaced striae. It has occasional breaks of striation laterally and a circular tail tip area lacking striae. The stylet, 15.8 μm long, has reniform knobs that merge gradually with the stylet shaft. Males have a high, rectangular, smooth head region, not set off from the body contour. The labial disc is continuous with the medial lips which do not slope posteriorly. The styler, 22.9 μm long, has large rounded backward sloping knobs; the shaft is of uneven diameter. Mean body length of second-stage juveniles is 453.6 μm. The truncate head region is not annulated, and the rounded, slightly raised labial disc and the crescentic medial lips form dumbbell-shaped lip structures. The stylet, 11.6 μm long, has rounded, posteriorly sloping knobs. The slender tail, 54.4 μm long, gradually tapers to a bluntly pointed tip. Tomato, tobacco, pepper, and watermelon are good hosts; cotton and peanut are not hosts. M. mayaguensis n. sp. reproduces by mitotic parthenogenesis and has a somatic chromosome number of 2n = 44-45. The enzyme patterns are unique among Meloidogyne species.     

Paternal Loss (PAL): A Meiotic Mutant in DROSOPHILA MELANOGASTER Causing Loss of Paternal Chromosomes

The effects of a male-specific meiotic mutant, paternal loss (pal), in D. melanogaster have been examined genetically. The results indicate the following. (1) When homozygous in males, pal can cause loss, but not nondisjunction, of any chromosome pair. The pal-induced chromosome loss produces exceptional progeny that apparently failed to receive one, or more, paternal chromosomes and, in addition, mosaic progeny during whose early mitotic divisions one or more paternal chromosomes were lost. (2) Only paternally derived chromosomes are lost. (3) Mitotic chromosome loss can occur in homozygous pal⁺ progeny of pal males. (4) Chromosomes differ in their susceptibility to pal-induced loss. The site responsible for the insensitivity vs. sensitivity of the X chromosome to pal mapped to the basal region of the X chromosome at, or near, the centromere. From these results, it is suggested that pal⁺ acts in male gonia to specify a product that is a component of, or interacts with, the centromeric region of chromosomes and is necessary for the normal segregation of paternal chromosomes. In the presence of pal, defective chromosomes are produced and these chromosomes tend to get lost during the early cleavage divisions of the zygote. (5) The loss of heterologous chromosome pairs is not independent; there are more cases of simultaneous loss of two chromosomes than expected from independence. Moreover, an examination of cases of simultaneous somatic loss of two heterologs reveals an asymmetry in the early mitotic divisions of the zygote such that when two heterologs are lost at a somatic cleavage division, almost invariably one daughter nucleus fails to get either, and the other daughter nucleus receives its normal chromosome complement. It is suggested that this asymmetry is not a property of pal but is rather a normal process that is being revealed by the mutant. (6) The somatic loss of chromosomes in the progeny of pal males allows the construction of fate maps of the blastoderm. Similar fate maps are obtained using data from gynandromorphs and from marked Y chromosome (nonsexually dimorphic) mosaics.     

Chromosome markers and alterations in mitotic cells from interspecific Citrus somatic hybrids analysed by fluorochrome staining

Mitotic cells from Rough lemon (Citrus jambhiri Lush.), Ohta ponkan (C. reticulata Blanco) and two somatic hybrid plants obtained from protoplast fusion were analysed by double staining with chromomycin A₃ (CMA) and 4′-6-diamidino-2-phenylindole. Only CMA-positive bands were observed in metaphasic chromosomes. The two parental karyotypes (2n=2x=18) were heteromorphic, yielding some marker chromosomes that could be identified in the somatic hybrids. One of the somatic hybrids had 2n=37 chromosomes, and the possible extra chromosome was distinguishable. The second somatic hybrid was tetraploid (2n=4x=36), with one of the chromosomes bearing a putative structural alteration. Furthermore, aneusomaty and some mitotic abnormalities were also observed in this latter plant. Such irregularities are reported for the first time for citrus somatic hybrids, and their possible causes and implications are discussed. Received: 23 December 1996 / Revision received: 21 May 1997 / Accepted: 16 June 1997     

Cytological investigations on two species of allocreadiid trematodes with special reference to the occurrence of triploidy and parthenogenesis in Allocreadium fasciatusi

Karyotypes and cytological details of gametogenesis were analysed for Allocreadium handiai and A. fasciatusi. Mitotic plates studied in squashes of testes, ovary, eggs and intramolluscan stages showed that A. handiai is a diploid (2n = 14) with six pairs of submetacentrics and one pair of subtelocentrics. Total chromosome length of the diploid complement was 86.57 μm, the largest chromosome measured 8.87 μm (10.24% total chromosome length [TCL]) and the smallest was 2.83 μm (3.28% TCL). Squashes of testes revealed the presence of all stages of spermatogenesis with spermatocytes in various stages of meiotic activity and spermatids containing bundles of spermatozoa. Stages of development of the ovum conclusively proved that reproduction takes place by amphimixis. Mitotic figures of A. fasciatusi, on the other hand, revealed that it is a triploid (3n = 21) with three metacentrics, 12 submetacentrics and six subtelocentrics. The mean total chromosome length of the triploid complement was 137.54 μm. The largest chromosome measured 10.37 μm (7.54% TCL) and the smallest measured 2.67 μm (1.94% TCL). Spermatogenesis was abortive with no evidence of synaptic pairing and spermatozoa were not produced. Eggs remained unfertilized and reproduction was achieved by parthenogenesis which is of mitotic type. The karyotypes of the two species differed not only in the ploidy level but also in the centromeric indexes of certain chromosomes.     

Karyotaxonomische Untersuchungen anPotentilla taurica s.l. in Bulgarie

Several taxa considered by previous authors as intraspecific ofPotentilla taurica s.l. are considered as separate species. A diploid chromosome number 2n=14 and the characteristic chromosomes of karyotypes of 3 Balkan endemicsP. ni?i?i Adam.,P. emilipopii Nyarády andP. pirotensis (Borb.) Mark. are recorded.P. bornmuelleri Borb.,P. mollicrinis (Borb.) Stankov andP. astracanica Jacq. are widespread in South-eastern Europe. Of this groupP. mollicrinis is a diploid (2n=14),P. bornmuelleri is tetraploid and inP. astracanica a polyploid serie of 2x=14, 4x=28, 5x=35 and 6x=42 has been found. The karyotypes of all species are characterized by submetacentric chromosomes.     

Chromosome mapping of repetitive sequences in four Serrasalmidae species (Characiformes)

The Serrasalmidae family is composed of a number of commercially interesting species, mainly in the Amazon region where most of these fishes occur. In the present study, we investigated the genomic organization of the 18S and 5S rDNA and telomeric sequences in mitotic chromosomes of four species from the basal clade of the Serrasalmidae family: Colossoma macropomum, Mylossoma aureum, M. duriventre, and Piaractus mesopotamicus, in order to understand the chromosomal evolution in the family. All the species studied had diploid numbers 2n = 54 and exclusively biarmed chromosomes, but variations of the karyotypic formulas were observed. C-banding resulted in similar patterns among the analyzed species, with heterochromatic blocks mainly present in centromeric regions. The 18S rDNA mapping of C. macropomum and P. mesopotamicus revealed multiple sites of this gene; 5S rDNA sites were detected in two chromosome pairs in all species, although not all of them were homeologs. Hybridization with a telomeric probe revealed signals in the terminal portions of chromosomes in all the species and an interstitial signal was observed in one pair of C. macropomum.     

Cytogenetic analysis of Lycopersicon esculentum (+) Solanum etuberosum somatic hybrids and their androgenetic regenerants

The aim of the study was to characterize genomic relationships among cultivated tomato (Lycopersicon esculentum Mill.) (2n=2x=24) and diploid (2n=2x=24) non-tuberous wild Solanum species (S. etuberosum Lindl.). Using genomic in situ hybridization (GISH) of mitotic and meiotic chromosomes, we analyzed intergeneric somatic hybrids between tomato and S. etuberosum. Of the five somatic hybrids, two plants were amphidiploids (2n=4x=48) mostly forming intragenomic bivalents in their microsporocytes, with a very low frequency of multivalents involving the chromosomes of tomato and S. etuberosum (less than 0.2 per meiocyte). Tomato chromosomes showed preferential elimination during subsequent meiotic divisions of the amphidiploids. Transmission of the parental chromosomes into microspores was also evaluated by GISH analysis of androgenic plants produced by direct embryogenesis from the amphidiploid somatic hybrids. Of the four androgenic regenerants, three were diploids (2n=2x=24 or 2n=2x+1=25) derived from reduced male gametes of the somatic hybrids, and one plant was a hypertetraploid (2n=4x+4=52). GISH revealed that each anther-derived plant had a unique chromosome composition. The prospects for introgression of desirable traits from S. etuberosum into the gene pool of cultivated tomato are discussed. Received: 2 August 2000 / Accepted: 4 December 2000     

The parthenogenetic Marmorkrebs (Malacostraca: Decapoda: Cambaridae) is a triploid organism

There is a close association between parthenogenesis and polyploidy. For this reason, we undertook a karyological analysis to test whether the parthenogenetic Marmorkrebs, Procambarus fallax forma virginalis, possesses an enlarged set of chromosomes. For this purpose, we karyotyped the Marmorkrebs, the sexual form of P. fallax (together called P. fallax complex), and the closely related species P. alleni. The latter shows 94 chromosomes in the haploid condition. In contrast to this, we found a haploid set of 92 chromosomes in individuals of the P. fallax complex. However, in mitotic metaphases the sexual form shows 184 chromosomes, whereas the Marmorkrebs possesses 276 chromosomes. Hence, the parthenogenetic Marmorkrebs reveals a triple amount of the haploid chromosome number. In addition, we detected a strikingly large subtelocentric chromosome which appears once in haploid and twice in diploid cells of sexual individuals of the P. fallax complex. In the parthenogenetic Marmorkrebs, this prominent chromosome occurs thrice. All this clearly reveals that the Marmorkrebs is a triploid organism. The applicability of the used methods, the significance of polyploidy in evolution of Decapoda, putative pathways to parthenogenetic triploidy, a possible hybrid origin and the scientific and ecological consequences of an increased chromosome set in Marmorkrebs are discussed.     

New chromosome numbers for plant taxa endemic to the Balearic Islands

Mitotic chromosome numbers are reported from 25 vascular plant taxa, endemic to the Balearic Islands that are poorly known cytogenetically. The chromosome numbers ofAnthyllis vulneraria subsp.balearica (2n=12),Cymbalaria fragilis (2n=56), andPolygonum romanum subsp.balearicum (2n=40) were determined for the first time. A new chromosome number was found in several populations ofAnthyllis hystrix (2n=70) suggesting that this species is decaploid, in contrast to an earlier work reporting a higher ploidy level (2n=12x=84). The new chromosome number 2n=32 was reported inHypericum hircinum subsp.cambessedesii. It is suggested that the previous count (2n=40) could be explained by the presence of anomalous pentaploid cells in some tissues, contrating with the presence of a regular tetraploid complement (2n=32). Cytogenetic observations suggest thatSibthorpia africana has a diploid chromosome complement of 2n=18, with 0–2 accessory chromosomes. Accessory chromosomes are also reported forPhlomis italica, being the first record of B chromosomes in this genus. Chromosomal instability was found inGalium crespianum andG. friedichii species, with three numbers 2n=44, 55 and 66. Two cytotypes differing in ploidy level were documented within single plants. It is suggested that both species share a regular complement of 2n=44 and that the past hybridization events and formation of regenerating roots from the typical rootstock ofG. crespianum andG. friedrichii could be involved in the genesis of chromosome variants through partial endopolyploidy and concomitant somatic segregation.