PARTHENOGENESIS, SEX AND CHROMOSOMES IN POTAMOPYRGUS

Widely different sex ratios in certain New Zealand populationsof the hydrobiid gastropod Potamopyrgus antipodarum (Gray) weregenerally found to vary little though in one stream a high-male/low-maleboundary moved steadily upstream over 20 years. Laboratory breeding and chromosome studies indicated that NewZealand high-male populations are diploid with 2n = 34 and reproducesexually, while New Zealand low-male populations and all thosefrom Europe and Australia have 2n = 52 or 46 and reproductionis largely parthenogenetic. (Received 15 July 1991; accepted 30 August 1991)     

Special Adaptive Problems Associated with Unisexual Fishes

SYNOPSIS. Two genera of fishes, Poecilia and Poeciliopsis, bothin the family Poeciliidae, have established unisexual populations.One "species," Poecilia formosa, is a diploid form that reliesupon sperm from Poecilia latipinna or P. mexicana to initiateparthenogenetic development (gynogenesis). Triploid individualsof P. formosa have also been discovered but the mode of reproductionand the significance of such fish to natural populations haveyet to be established. In the genus Poeciliopsis three gynogenetic triploids are known.Each is superimposed on a different bisexual host: Poeciliopsislucida, P. monacha or P. viriosa. Five diploid hybridogeneticformsalso have been identified. These are fertilized by either P.lucida, P. latidens, or P. occidentalis. In hybridogenetic forms,sperm actually feitilizes the egg. A hybrid with maternal andpaternal characters is produced, but only maternal genes aretransmitted through the egg to the next generation. Because of their sperm dependency, all unisexual fishes mustreside with at least one of the parental species. They areforcedto compete with them for sperm, space, and other limited resources.The ability to produce two females for every one that bisexualsproduce suggests that the unisexuals have the potential to winthis competition; but in so doing, they would lose their sexualsupport and eliminate themselves. A density-dependent mate selectionsystem plays a role in establishing a workable balance betweenthe competing elements. Unisexual forms of both Poecilia and Poeciliopsis are of hybridorigin. For most of them, the parental precursors and the proportionof genetic material that each contributed to the unisexual isknown. The various genomes that make up the unisexuals are adaptedto a variety of habitats. Once these genomes are combined ina unisexual hybrid, potential for adaptation to multiple nichesexists; this coupled with heterosis contributes greatly to thesuccess of gynogenetic and hybridogenetic fishes.     

Amylolytic Activity in Taproots of Diploid and Tetraploid Medicago sativa L.

Our objective was to determine whether starch degradation intaproots of alfalfa (Medicago sativa L.) after defoliation wasassociated with activity and isoform complement of endo- andexo-amylases. Taproots of genetically comparable diploid (2x)and tetraploid (4x) populations and the tetraploid cultivarHi-Phy were recovered immediately after defoliation and at approx.4-d intervals thereafter. Taproot tissues were analysed forstarch concentrations and activities of endo- and exo-amylases.An electrophoretic blotting technique was used to examine amylaseisoforms. Starch degradation was most rapid in taproots of Hi-Phy,slowest in taproots of the 2x population, with the 4x populationbeing intermediate. The 4x population had a greater initialincrease in endo-amylase activity compared to the 2x population;however, Hi-Phy averaged eightfold greater endo-amylase activitythan either 2x or 4x populations. Although exo-amylase specificactivity was at least 500-fold greater than endo-amylase specificactivity in all populations, changes in endo-amylase activitywere more closely associated with trends in starch degradation.Multiple isoforms of endo- and exo-amylase were observed intaproots of all populations. Taproots of Hi-Phy contained anendo-amylase isoform that was not apparent in the 2x or 4x populationsthat may contribute to the high activity of this amylase intaproots of this cultivar. These results, although correlative,suggest an important role for endo-amylase in taproot starchhydrolysis after defoliation. Medicago sativa (L.), alfalfa, taproots, herbage, diploid, tetraploid, starch, endo-amylase, exo-amylase, isoforms, electrophoresis     

GENETIC AND MORPHOLOGICAL VARIATION OF POPULATIONS BELONGING TO THE BULINUS TRUNCATUS/TROPICUS COMPLEX (GASTROPODA; PLANORBIDAE) IN SOUTH WESTERN ZIMBABWE

Aquatic snails from south western Zimbabwe belonging to theBulinus trunscatus/tropicus complex vary widely in shell formsuggesting that more than one taxon could be present. This possibilitywas investigated by making observations on snail samples from13 populations from the Plumtree area, in respect of allozymevariation (5 polymorphic loci), shell morphology (9 variables),copulatory organ and chromosome number. Comparative data wereobtained from snails from north western Zimbabwe identifieddefinitely as B. tropicus. Analysis of the genetic structurerevealed a high degree of polymorphism (P) ranging from 0.29–0.80among populations from Plumtree and expected heterozygosity(H_e) from 0.02–0.22. No enzymatic diagnostic loci werefound which could differentiate the different morphs or populationsand discriminant function analysis on the morphological datashowed an overlap of morphs among populations. Snails analyzedfor chromosome number were all diploid (2n = 36). Snails exposedto Schistosoma haematobium mira-cidia were all refractory. Thisinformation supports the view of a single species, B. tropicus,which is differentiated due to migration barriers and whereenvironmental variables might be implicated in the morphometricdivergence. (Received 31 July 1995; accepted 15 January 1998)     

The Influence of Seed Size Variation on Seed Germination and Seedling Vigour in Diploid and Tetraploid Dactylis glomerata L.

In this study the effects of seed size variation on germinationand seedling vigour have been investigated within and betweenploidy levels of diploid and related autotetraploid Dactylixglomerata. Rates of seed germination and seedling growth werecompared in two contrasting environments using diploid and tetraploidseeds of equal and also different biomass. Within each ploidylevel, seed biomass had no effect on either the overall percentagenor the rate of germination. In contrast, the comparison ofseeds of equal biomass but differing in ploidy level showedthat seeds from tetraploid plants germinated faster and to ahigher percentage than those from diploid plants. With respectto seedling growth, heavier seeds from the tetraploid genotypesgave seedlings of significantly higher biomass than those fromlighter tetraploid and diploid seeds throughout the 2 monthsof study. Interestingly, seeds of equal biomass but from differentploidy levels produced seedlings more similar than those fromthe extreme seed weight categories. These differences were maintainedin two different environmental conditions. These results suggestthat there is a complex interdependance of seed size and ploidyon seed germination and seedling growth but is not a simpleconsequence of differences in seed size between diploids andtheir related tetraploids.Copyright 1995, 1999 Academic Press Dactylis glomerata, polyploidy, seed size, germination, seedling     

Chromosome Number in Cynodon dactylon in Relation to Ecological Conditions

Populations of Cynodon dactylon (L.) Pers. were collected fromthree habitat types (roadsides, lawns and paddy-fields) withineach of five climatic regions (arid, dry, intermediate, wetand hill country) in Sri Lanka. Further populations were collectedfrom forests and grasslands in the hill country. Most populations contained only tetraploid plants (2n = 36),but populations from roadsides and lawns in the wet region,and from forests in the hill country, contained only diploidplants (2n = 18). Populations from paddy-fields in the wet regioncontained both tetraploid and diploid plants. No triploid plantswere found in any environment, despite the fact that C. dactylonpredominantly reproduces vegetatively by both stolons and rhizomes.Soil acidity seems to be the main factor determining the distributionof the two cytotypes.Copyright 1995, 1999 Academic Press Cynodon dactylon (L.) Pers., chromosome number, ecology, habitat, climatic region     

Morphological characterization of adult Artemia (Crustacea, Branchiopoda) from different geographical origins. American populations

The morphological characterization of adult Artemia femalesbelonging to 25 different populations from all over the Americashas been accomplished. The measures of 12 different parametershave been taken from individuals of each population raised understandardized culture conditions from nauplii to adult. Theyhave been analyzed through multivariate discriminant proceduresin order to establish relationships among the different populations,as well as to have a tool to assign new sets of data for unknownpopulations to one of the groups analyzed here. We have beenable to clearly separate the populations of the two differentspecies analyzed (Artemia persimilis and Artemia franciscana).In addition, we distinguish three different clusters of populationsamong the A.franciscana populations: North American athalassicinland populations, individuals from the Caribbean coast areaand populations from the Pacific Ocean shore.     

MATING SYSTEM ANALYSIS USING PROTEIN ELECTROPHORESIS IN THE SELF-FERTILE HERMAPHRODITE SPECIES BULINUS TRUNCATUS (GASTROPODA: PLANORBIDAE)

Uniparental reproduction in the tetraploid hermaphrodite speciesBulinus truncatus has been suggested to occur via self-fertilization,on the basis of cytological and genetic studies. However, recentanalyses of population genetic structure by protein electrophoresisindicated the occurrence of multi-banded phenotypes referredto as ‘fixed hetero-zygosity’. Although intrapopulationand geographical variation of multibanded phenotypes occur,many populations bear only one type of such patterns. This ledto the suggestion that parthenogenesis may well be the matingsyste in B. truncatus. However, such fixed heterozygosity patternsare expected in some tetraploid species in which double disomicinheritance of alleles occurs. It is therefore not possibleto determine from population genetic structure analysis alonewhether segregation occurs or not. Here, we investigate thestability of such patterns over one generation of uniparentalreproduction among four populations. We also present resultsof crossing experiments between individuals from two populations,using two diagnostic loci to analyse their offspring. Our resultsclearly indicate Mendelian segregation of alleles, and confirmsexual reproduction by self-fertilization and cross-fertilization.We interpret the multibanded patterns observed in populationsas the product of both diploid loci of the tetraploid genomewhen they are monomorphic for different alleles. Our study alsoallows us to suggest that partial selfing may be the regularmating system in B. truncatus. (Received 12 February 1992; accepted 7 September 1992)     

Widespread Chromosome Variation in the Endangered Grassland Forb Rutidosis leptorrhynchoides F. Muell. (Asteraceae: Gnaphalieae)

Rutidosis leptorrhynchoides is an endangered plant endemic tosoutheastern Australia. Chromosome analysis of 19 of the 24known populations of the species has identified 17 differentchromosome variants or cytotypes. The most common cytotypesare a diploid and a tetraploid based on x = 11, and triploidand hexaploid plants with this basic number were also observed.Diploids, triploids and tetraploids based on a second basicnumber ofx = 13 were also seen. Plants with 2 n = 24 were shownto be hybrids between diploids with the two different basicnumbers. Meiotic chromosome pairing analysis of the plants with2n = 24 showed a maximum of two trivalents indicating the presenceof extra copies of one pair of large and one pair of small chromosomesin the 2 n = 26 plants. In addition, a number of different aneuploidsof the 2 n = 22 and 2 n = 44 races were found and many of thesealso showed structural chromosomal variation. The distributionof the two main chromosome races is disjunct with the tetraploidsconfined to southern Victoria. To avoid dysgenic effects, futurere-establishment efforts for this species should avoid mixingseed from different chromosome races. Copyright 2001 Annalsof Botany Company Aneuploidy, conservation genetics, karyotypes, meiosis, polyploidy     

The Diatom Genus Tabellaria: II. Taxonomy and Morphology of the Plankton Varieties

Plankton samples of Tabellaria from all the lakes of the EnglishLake District have been examined and evidence of morphologicallydistinct populations has been found in samples from (a) a singlelake at one time, (b) different lakes, and (c) a single lakeat different times. Clone cultures have been used to study thenature of these differences. Fifteen characters have been foundto show quantitative or qualitative variation in plankton populationsof Tabellaria, and the morphological diversity of differentpopulations is largely due to different combinations of thesecharacters. All known plankton populations of Tabellaria arereferred to the species T. flocculosa.     

Variation of Nuclear DNA Content in the Biflorus Species of Lonchocarpus (Leguminosae)

This represents the first study of nuclear DNA content in alarge sample (135 spp.) from a tropical arboreal genus, in whicha large proportion of the species were examined (42 spp., 31.1%).Somatic chromosome numbers and 4C-DNA values for 51 taxa ofLonchocarpus are reported. All taxa were diploid with 2 n =22,but their DNA content ranged from 1.92 to 2.86 pg 4C nucleus,corresponding to a 48.95% variation in genome size. In the 74collections studied, no correlation was observed between DNAcontent and habitat altitude. Variation in nuclear DNA contentwas analysed at the level of genus, subgenus, section and subsection.Variation in genome size was also studied within some species,either among widely separated populations or among differentintraspecific taxa. Very little variation in genome size wasdetected between populations, subspecies, and varieties of thesame species. The taxonomic implications of variation in nuclearDNA content are discussed.Copyright 2000 Annals of Botany Company Lonchocarpus (Leguminosae), DNA content, chromosome number.     

The Relationship between Embryo and Endosperm in Interspecific Hybrids in PennisetumL. Rich

In the interspecific cross [diploid Pennisetum typhoides X hexaploid(P. typhoides X P. purpureum)] X diploid P. typhoides, embryosand endosperms in the same ovule may respond differently totreatment with the growth substance gibberellic acid, as maydo different genotypes. Size of embryo and size of its endospermare positively correlated in some genotypes but not in others.Large fully developed normal embryos may be associated withnormal, retarded, degenerate or even no endosperm at all. Conversely,large normal endosperms, even in mature seeds, may contain large,small, or no embryos. Fully developed embryos may be viableregardless of whether they had been associated with normal,retarded or degenerate endosperm, or even no endosperm at all. It is concluded that the embryo is not always dependent on theendosperm during its early development and that a degeneratingendosperm need not have a detrimental effect on the developmentof its embryo or on the viability of the latter.     

Structure, Life-history, and Sex Determination of Prasiola stipitata Suhr

By means of cultures and cytological investigations P. stipitatawas shown to have a dimorphic diplo-haploidic life-history,as follows: The diploid spore-forming thalli produce diploidspores (2n = 12(14)). The sexual plants comprise a diploid meiosporophyteand numerous haploid gametophytes (n = 6(7)). Sex determinationis haplogenotypic.     

Effect of Sporophytic Heterozygosity on the Male Gametophyte of the Tetraploid Potato (Solanum tuberosum L.)

Quantitative variation for pollen stainability was examinedin both tetraploid x diploid and tetraploid x tetraploid familiesof potato (Solanum tuberosum L.). The diploid parents, whichwere highly heterozygous, produced first division restitution(FDR) 2n pollen. The families used were not affected by cytoplasmic-geneticmale sterility. Significant differences between families werefound. FDR 2n pollen producing diploid clones had progenieswith a significantly higher pollen stainability than those derivedfrom crosses between tetraploid parents. This was related tothe level of heterozygosity in the sporophyte. In FDR 2n pollen,80% of the heterozygosity and a large fraction of epistasispresent in the diploid parent was transmitted to its tetraploidprogeny. This is important for the production of open pollinated,true potato seed, because pollen stainability is significantlycorrelated with fruit set (r = 0·85).Copyright 1994,1999 Academic Press FDR 2n pollen, gametophytic heterozygosity, true potato seed     

Endopolyploidy in Diploid and Tetraploid Maize (Zea mays L.)

Nuclei from different tissues such as stem, mesocotyl, nodalroot and root tip of diploid and tetraploid maize were isolated,stained with propidium iodide and passed through an EPICS-751flow-cytometer cell sorter. Variations in flow histograms wereobserved in different tissues. Stem tissues of both the diploidand tetraploid had two peaks representing G1 and G2 somaticnuclei. The remaining tissues in both the diploids and tetraploidsexhibited three peaks. The first peak observed in these tissuesrepresents G1 somatic nuclei of the lowest ploidy level. Thesecond peak represent G2 somatic nuclei of the lowest ploidylevel+G1 somatic nuclei of the next ploidy level. The thirdpeak represents G2 of the higher ploidy level+G1 somatic nucleiof the next higher ploidy level. Statistically significant differenceswere observed between the diploid and tetraploid maize tissueswith respect to nuclei distribution in the higher ploidy levelpeaks implying variation in the degree of endopolyploidy inthe diploid and tetraploid maize. The results of this studysuggest that the amount of endopolyploid observed in maize genotypeshas an effect on their overall agronomic performance under thefield conditions.Copyright 1993, 1999 Academic Press Zea mays L., maize, endopolyploidy, diploid, tetraploid, flow cytometry     

Comparison of Embryo Development in Wild and Cultivated Arachis Species

Embryo development following selfing was investigated in twowild diploid peanut species, Arachis batizocoi Krap. et Greg.(coll. K 9484) (2n = 20) and A. duranensis Krap. et Greg. nom.nud. (coll. K 7988) (2n = 20), and one cultivated tetraploidspecies, A. hypogaea L. NC-Ac 18000 (2n = 40). Rates of pegelongation and sequences of embryo development for each specieswere compared. Peg elongation rates were similar for the twowild species, but for A. hypogaea it was only one-third to one-halfthat of the diploid species. Embryos in A. hypogaea showed slightlymore rapid cell division than in the wild species. The observedvariation in reproductive development between the wild and cultivatedspecies indicate that different control mechanisms may governdevelopment in the different species and may be at least partiallyresponsible for failure to produce viable interspecific hybridsat various ploidy levels. The observations are also importantfor determining the time at which embryos of different speciesof Arachis will reach the appropriate stage of development forsuccessful culture on an artificial medium during embryo rescueprocedures. Peanut, Arachis hypogaea, wild species, embryo, peg     

Electrophoretic mobility of haploid cells, diploid cells, asci, and ascospores of Saccharomyces cerevisiae

The electrophoretic mobility of diploid cells, haploid cellsof different mating types, their cell walls, asci, and ascosporesof Saccharomyces cerevisiae was measured with a free-flow electrophoreticapparatus. Haploid -cells and ascospores exhibited higher mobilitythan diploid cells, haploid -cells, and asci. Similar differencesin mobility were found with isolated cell walls of the haploidand diploid cells. ² Present address: Lederle (Japan) Ltd., Kyobashi, Tokyo 104,Japan. (Received December 24, 1976; )     

Cytological Studies in the Laminariales

A cytological investigation of British members of the orderLaminariales has revealed the presence of a very large X-chromosomein dividing nuclei of female gametophytes and young sporophytesof Saccorhiza polyschides. This chromosome pairs with a smallY-chromosome at meiosis. A chromosome which is conspicuouslylarger than the others has also been detected in the femalegametophytes (and young sporophytes) of Laminaria spp., Chordafilum, and Alaria esculenta. There is some evidence that thisalso pairs with a smaller chromosome. Chromosome numbers determinedduring the work are as follows: Laminaria digitata, L. saccharina,L. hyperborea, L. ochroleuca, Saccorhiza polyschides, hap-loid31 and diploid 62; Alaria esculenta, haploid 28 and diploid56; Chorda filum, haploid c. 28 and diploid c. 56     

ALLOZYME COMPARISONS AND PLOIDY LEVELS AMONG SPECIES OF THE BULINUS TRUNCATUS/TROPICUS COMPLEX (GASTROPODA: PLANORBIDAE) IN CAMEROON

Allozyme electrophoresis, ploidy level, and, to a lesser extent,anatomy and shell morphology were used to characterize snailsof the Bulinus truncatus/tropicus complex in Cameroon. Starchgel electrophoresis revealed no intrapopulation variation within20 populations examined for 14 allozyme loci in 12 enzyme systems.Interpopulation differences in allozyme patterns were used todivide the populations studied into five groups. Each groupis distinguished by a unique combination of zymograms for hydroxybutyratedehydrogenase and isocitrate dehydrogenase. Four of these groupsare tetraploids and are identified as B. truncatus. Populationsof the most commonly found B. truncatus group are generallyaphallic, are frequently associated with transmission of S.haematobium, and generally occur at altitudes below 800 m abovesea-level. Populations of B. truncatus belonging to the otherthree groups are usually euphallic, not associated with S. haematobiumtransmision, and generally occur at altitudes above 1000 m.These four groups may represent distinct biotypes of B. truncatus.Populations belonging to the fifth group are diploid, euphallic,not associated with S. haematobium transmission, and occur inhabitats above 1000 m. This is the first confirmed report ofdiploid B. truncatus/tropicus-complex snails from this regionof Africa. They are tentatively identified as B. natalensis,a species previously thought to be limited to East and SouthernAfrica. (Received 31 October 1988; accepted 20 February 1989)     

Natural variation in Arabidopsis lyrata vernalization requirement conferred by a FRIGIDA indel polymorphism

Species share homologous genes to a large extent, but it isnot yet known to what degree the same loci have been targetsfor natural selection in different species. Natural variationin flowering time is determined to a large degree by 2 genes,FLOWERING LOCUS C and FRIGIDA, in Arabidopsis thaliana. Here,we examine whether FRIGIDA has a role in differences in floweringtime between and within natural populations of Arabidopsis lyrata,a close outcrossing perennial relative of A. thaliana. We found2 FRIGIDA sequence variants producing potentially functionalproteins but with a length difference of 14 amino acids. Thesevariants conferred a 15-day difference in flowering time inan association experiment in 2 Scandinavian populations. Thedifference in flowering time between alleles was confirmed withtransformation to A. thaliana. Because the north European late-floweringpopulations harbor both late- and early sequence variants atintermediate frequencies and the late-flowering variant is mostfrequent in the southern early flowering European population,other genetic factors must be responsible for the floweringtime differences between the populations. The length polymorphismoccurs at high frequencies also in several North American populations.The occurrence of functional variants at intermediate frequenciesin several populations suggests that the variation may be maintainedby balancing selection. This is in contrast to A. thaliana,where independent loss-of-function mutations at the FRIGIDAgene are responsible for differences between populations andlocal adaptation.