Parasite recruitment by stocked walleye, Stizostedion vitreum vitreum (Mitchill), fry in a small boreal lake in central Canada

Six species of parasites were recovered 4 mo after walleye fry were stocked in Heming Lake, Manitoba. The species of parasites acquired most rapidly were those that were non-host-specific and common to the indigenous populations of both walleye and yellow perch (Perca flavescens). Parasite species overlap (Jaccard's indices) was greatest within age groups of walleye and yellow perch, but was also high between older walleye and yellow perch. The higher numbers of parasites recruited by stocked walleye, particularly ones known to induce pathology, raises questions on the success of walleye introductions to aquatic systems with a diverse indigenous parasite fauna and a fish population with a large proportion of yellow perch.     

Induction of gynogenesis in muskellunge with irradiated sperm of yellow perch proves diploid muskellunge male homogamety

Diploid gynogenesis was induced in muskellunge Esox masquinongy using UV-irradiated muskellunge sperm as the first step in producing monosex females. In this approach, we have to rely on negative controls as an indirect reference for sperm genetic material destruction. In the first experiment, equal proportions of gynogenetic females and males were produced. Negative controls, UV-irradiated sperm without heat shock, yielded some normal hatching larvae, described as spontaneous diploids. In the second experiment, muskellunge eggs were activated using sperm from yellow perch. Because hybrids between these species are not viable, we produced unambiguous gynogens. When UV-irradiated yellow perch sperm was used to inseminate muskellunge eggs, haploids resulted (22.5% +/- 2.8% survival to the eyed stage). To produce diploid gynogens, a heat shock of 31 degrees C was applied to inseminated eggs 20 min after activation for a duration of 6 min. This process yielded several hundreds of gynogens for rearing. Several treatments of masculinizing hormone, 17alpha-methyltestosterone (MT), were carried out. Fish were dissected and gonads examined histologically for sex determination. Gynogens produced using yellow-perch sperm confirmed the presence of males in the control group, whereas the MT bath treatment (400 microg/liter) resulted in the production of fish with ovotestis. These results provide evidence for male homogamety in muskellunge and imply that a change of strategy is needed to produce monosex populations.     

Lazy males? Bioenergetic differences in energy acquisition and metabolism help to explain sexual size dimorphism in percids

1. Differences in energy use between genders is a probable mechanism underlying sexual size dimorphism (SSD), but testing this hypothesis in the field has proven difficult. We evaluated this mechanism as an explanation for SSD in two North American percid species--walleye Sander vitreus and yellow perch Perca flavescens. 2. Data from 47 walleye and 67 yellow perch populations indicated that SSD is associated with the onset of maturation: typically, males of both species matured smaller and earlier and attained a smaller asymptotic size than females. Males also demonstrated equal (perch) or longer (walleye) reproductive life spans compared with females. 3. To examine whether reduced post-maturation growth in males was due to lower energy acquisition or higher reproductive costs we applied a contaminant mass-balance model combined with a bioenergetics model to estimate metabolic costs and food consumption of each sex. Mature males exhibited lower food consumption, metabolic costs and food conversion efficiencies compared with females. 4. We propose that slower growth in males at the onset of maturity is a result of decreased feeding activity to reduce predation risk. Our finding that SSD in percids is associated with the onset of maturity is supported by laboratory-based observations reported elsewhere, showing that changes in growth rate, consumption and food conversion efficiency were elicited by oestrogen (positive effects) or androgen (negative effects) exposure in P. flavescens and P. fluviatilis. 5. Researchers applying bioenergetic models for comparative studies across populations should use caution in applying bioenergetic models in the absence of information on population sex ratio and potential differences between the sexes in energetic parameters.     

Effect of triploid fitness on the coexistence of diploids and tetraploids

The conditions for the coexistence of diploids, triploids and tetraploids in a single population were investigated with a deterministic model under the assumptions that diploids might produce 2 n gametes, and that triploids had a lower fitness than other cytotypes and generated equal proportions of haploid and diploid gametes. When diploids produced only haploid gametes, the dynamics of the cytotypes were similar to that of heterozygote disadvantage with two alleles at a single locus, with triploids being equivalent to the heterozygotes. Production of 2 n gametes by diploids increased the pool of diploid gametes and created a stable equilibrium involving a majority of diploids and a minority of polyploids. When the fitness of tetraploids was equal to or higher than that of diploids, increased triploid fitness decreased the threshold of 2 n gametes necessary to deterministically fix tetraploids in the population. Conversely, when tetraploids were less fit than diploids, the rate of 2 n gamete production leading to the exclusion of diploids first decreases and then increased with increasing triploid fitness. Triploids are repeatedly found in diploid-tetraploid hybridizations and are rarely totally sterile. They might play a determinant role in the future of multiple cytotype populations. The effect of triploids depends on the relative fitness of diploids and tetraploids and is also a function of their fitness.     

Polyploid progeny from triploid hybrids of Phegopteris decursivepinnata (Thelypteridaceae)

Journal of Plant Research - Phegopteris decursivepinnata includes diploids, tetraploids, and triploid hybrids based on x = 30. We obtained polyploid progeny from triploid hybrids...     

Evidence of predatory control of yellow perch (Perca flavescens) recruitment in Lake Erie, U.S.A.

Predation can be a major factor in recruitment success of yellow perch ( Perca flavescens Mitchitl). Trawl catches of age-0 yellow perch in western Lake Erie declined from 870·3 per trawling h in June to 3·3 per trawling h in late July 1988. Coincident with the decline in relative abundance of age-0 yellow perch we found large numbers of age-0 yellow perch in the stomachs of small walleyes ( Stizostedion vitreum Mitchill). From this evidence we hypothesized that predation by walleyes may have caused the demise of the 1988 year-class of yellow perch. We used a population and bioenergetics modelling approach to estimate the impact of walleye predation on the abundance of age-0 yellow perch. Modelling showed that 6·8 × 10⁹ age-0 yellow perch that had attained 18 mm total length ( t.l. ), were eaten by small (age-2 and younger) walleye from June through July 1988. We estimated that walleye ate 28·4–89·7% of the yellow perch reaching 18 mm t.l. during 1988. The majority of this predation (77% of total) was by the abundant age-2 cohort of walleyes. We concluded that, even in a large system such as Lake Erie, predation can play a major role in structuring year-class strength of yellow perch and, thus, management of percid fisheries should be conducted on a fish-community basis.     

Use of Biochronology to Examine Interactions of Freshwater Drum, Walleye and Yellow Perch in the Red Lakes of Minnesota

Growth histories for freshwater drum, Aplodinotus grunniens, walleye, Stizostedion vitreum, and yellow perch, Perca flavescens, were constructed using calcified structures for the period 1947 through 1996 for the Red Lakes, Minnesota. Increased walleye growth and decreased yellow perch growth were observed over the period from 1983 to 1996, which are attributed to intensive fishing resulting in decreased intraspecific competition in walleye and increased intraspecific competition in yellow perch through release from predation. Strong year-classes of yellow perch were positively correlated with walleye growth (r=0.57, p-value=0.042). There was no evidence for interactions of walleye or yellow perch with freshwater drum. Freshwater drum growth (r=0.680, p-value=0.0001) was more highly correlated with temperature than were walleye (r=0.386, p-value=0.006) and yellow perch growth (r=0.303, p-value=0.036).     

Pollen competition as a unilateral reproductive barrier between sympatric diploid and tetraploid Chamerion angustifolium

Speciation requires the evolution of barriers to gene exchange between descendant and progenitor populations. Cryptic reproductive barriers in plants arise after pollination but before fertilization as a result of pollen competition and interactions between male gametophytes and female reproductive tissues. We tested for such gametic isolation between the polyploid Chamerion angustifolium and its diploid progenitor by conducting single (diploid or tetraploid) and mixed ploidy (1 : 1 diploid and tetraploid) pollinations on both cytotypes and inferring siring success from paternity analysis and pollen-tube counts. In mixed pollinations, polyploids sired most (79%) of their own seeds as well as those of diploids (61%) (correcting for triploid block, siring success was 70% and 83%, respectively). In single donor pollinations, pollen tubes from tetraploids were more numerous than those from diploids at four different positions in each style and for both diploid and tetraploid pollen recipients. The lack of a pollen donor x recipient interaction indicates that the tetraploid siring advantage is a result of pollen competition rather than pollen-pistil interactions. Such unilateral pollen precedence results in an asymmetrical pattern of isolation, with tetraploids experiencing less gene flow than diploids. It also enhances tetraploid establishment in sympatric populations, by maximizing tetraploid success and simultaneously diminishing that of diploids through the production of inviable triploid offspring.     

Morphometrical and acoustical comparison between diploid and tetraploid green toads

In the present paper we compare, on the basis of morphometrical characters and acoustical properties of the advertisement calls, a sample of 158 male green toads (Bufo viridis complex) collected in 12 breeding populations of south Kazakstan and north Kyrgyzstan. The samples of three populations resulted in only diploid toads (2n = 22), those of eight populations in only tetraploid toads (2k=44) whereas in one locality diploid, tetraploid and many triploid toads were collected. Diploid toads show significantly larger body size and proportionally larger head and shorter limbs than both tetraploids and triploids, whereas no evident morphometrical differences were observed between triploids and tetraploids. Diploid advertisement calls have spectral and temporal properties that significantly differ from those of both triploid and tetraploid advertisement calls. In particular, diploids produce significantly longer calls with higher pulse-rates and lower frequencies than those of tetraploids. We address the question of the factors that could be responsible for these differences and we discuss four hypotheses: (1) the direct effect of polyploid mutation, (2) genetic drift, (3) reproductive character displacement and (4) environmental selection.     

黄姑鱼三倍体的诱导、鉴定及其性腺发育特征

研究采用冷休克方法诱导黄姑鱼(Nibea albiflora)三倍体并进行鉴定,进一步采用组织学和生殖相关基因表达分析比较了三倍体黄姑鱼的性腺发育特征。研究结果表明:(1)在受精后2.5min以3℃进行10min的冷休克处理,冷休克处理组的受精率和孵化率分别为(70.31±4.49)%和(21.5±6.63)%,其受精率和孵化率显著低于二倍体对照组;(2)经流式细胞仪倍性检测和染色体核型分析发现,三倍体的DNA含量为二倍体的1.5倍,染色体数目为72条,而二倍体的染色体数目为48条,三倍体的比例为100%;(3)三倍体的生殖腺指数显著低于二倍体,进一步通过组织切片观察发现三倍体的精巢和卵巢发育较二倍体滞后,在12月龄时,二倍体精巢和卵巢处于Ⅴ期,而三倍体精巢和卵巢分别处于Ⅲ期和Ⅰ期;(4)三倍体精巢的dmrt1和vasa基因及三倍体卵巢的cyp19a基因表达量均显著低于二倍体(P<0.05);三倍体卵巢的vasa基因表达量也比二倍体低(P>0.05)。综上结果表明:研究通过冷休克处理成功诱导了黄姑鱼三倍体;三倍体的性腺发育较二倍体滞后,育性明显降低。     

Cytotype distribution inEmpetrum (Ericaceae) at various spatial scales in the Czech Republic

Ploidy levels inEmpetrum (crowberry) from the Czech Republic and from one adjacent locality in Poland were estimated by flow cytometry to examine cytotype distribution patterns at large (within the country), medium (within mountain ranges) and small (within particular localities) spatial scales. Diploid, triploid, and tetraploid individuals were found. Triploids are reported from Central Europe for the first time; they occurred in the Krkono?e Mts. Exclusively diploid plants were observed in three mountain ranges (the Kru?né hory Mts., Labské pískovce Mts., Adr?pa?sko-Teplické skály Mts.), exclusively tetraploids were observed in the Jeseníky Mts., and both cytotypes were observed in the ?umava Mts., Jizerské hory Mts. and Krkono?e Mts. Except for the latter mountain range, diploids and tetraploids were always found in different habitats. Spatial isolation is supposed to be the main barrier preventing cytotype mating. A mosaic-like sympatric occurrence of different cytotypes was demonstrated in the Krkono?e Mts., where peat bogs and rocky places were not spatially separated. Eight of 11 localities studied there were inhabited by diploids and tetraploids (five localities), diploids and triploids (one locality) or all three ploidy levels (two localities). Diploid and triploid plants occasionally intermingled at 0.3 × 0.3 m. Flower sex in crowberries was strongly associated with ploidy level: diploids usually had unisexual flowers, the tetraploids bore exclusively bisexual flowers. However, a few diploid plants with hermaphrodite flowers occurred in one population in the Krkono?e Mts.     

Habitat differentiation, hybridization and gene flow patterns in mixed populations of diploid and autotetraploid Dactylorhiza maculata s.l. (Orchidaceae)

Detailed ecological, morphological and molecular analyses were performed in mixed populations of diploid and autotetraploid Dactylorhiza maculata s.l. in Scandinavia. Comparisons were made with pure populations of either diploid ssp. fuchsii or tetraploid ssp. maculata. It was shown that mixed populations are the result of secondary contact between ssp. fuchsii and ssp. maculata. No patterns of recent and local autopolyploidization were found. Morphology and nuclear DNA markers (internal transcribed spacers of nuclear ribosomal DNA) showed that diploids and tetraploids from mixed populations have similar levels of differentiation to diploids and tetraploids from pure populations. Vegetation analyses, as well as analyses of environmental variables, revealed that diploid and tetraploid individuals in mixed populations are ecologically well differentiated on a microhabitat level. Diploids and tetraploids in pure populations have wider ecological amplitudes than they do in mixed populations. Triploid hybrids grew in intermediate microhabitats between diploids and tetraploids in the mixed populations. Plastid DNA markers indicated that both diploids and tetraploids may act as the maternal parent. Based on morphology and nuclear markers triploids are more similar to tetraploids than to diploids. There were indications of introgressive gene flow between ploidy levels. Plastid markers indicated that gene flow from diploid to tetraploid level is most common, but nuclear markers suggested that gene flow in opposite direction also may occur. Similar patterns of differentiation and gene flow appeared in localities that represented contrasting biogeographic regions. Disturbance and topography may explain why hybridization was slightly more common and the differentiation patterns somewhat less clear in the Scandinavian mountains than in the coastal lowland. An erratum to this article can be found at     

Variations in hepatic estradiol-17beta receptor concentrations during the annual reproductive cycle of diploid and triploid female catfish, Heteropneustes fossilis (Bloch)

Concentrations of hepatic estradiol-17beta (E2) receptors (ER) in cytosolic and nuclear fractions were evaluated in diploid and triploid female catfish Heteropneustes fossilis (Bloch) during four different reproductive periods of a complete reproductive cycle. Basal level of ER concentration was noted in the resting period of both diploids and triploids. Receptor level gradually elevated through the preparatory period and reached a peak in the pre-spawning period in both diploids and triploids. However, ER concentrations were overall reduced in triploid to that of diploid females. In a single point assay, in diploids, ER concentration showed about a 3-fold rise (p<0.001) in the cytosolic and a 4-fold rise (p<0.001) in the nuclear extracts from resting to the pre-spawning period. In triploids, only a 2-fold rise was observed both in cytosolic (p<0.01) and nuclear (p<0.05) ER concentration during the same span. Finally, a sudden fall of receptor level was observed in the spawning period in both the ploidy groups with a lower concentration in the triploids. The K(d) value did not differ between the females of diploids (cytosolic 1.12+/-0.21 nM and nuclear 6.9+/-0.9 nM) and triploids (cytosolic--1.13+/-0.17 nM, nuclear--6.8+/-2 nM). However, B(max) of the diploid showed about double the value than triploid females both in the cytosolic (diploid--367.4+/-33.24 pmol/mg protein, triploid--187.3+/-13.20 pmol/mg protein, p<0.001) and nuclear extracts (diploid--946+/-66 pmol/mg DNA, triploid-558+/-98 pmol/mg DNA, p<0.01) of liver. Lower E2 binding capacity and lower amount of E2 receptors of triploid catfish liver with a stunted vitellogenic status could be one of the major causes for reduced gonadal development and sterility in female triploids.     

A secondary hybrid zone between diploid Dactylorhiza incarnata ssp. cruenta and allotetraploid D. lapponica (Orchidaceae)

Secondary hybrid zones are not uncommon in Dactylorhiza, but knowledge of ecological and evolutionary consequences of hybridization are scarce. Here, we assess interploidal gene flow and introgression in a hybrid zone between diploid Dactylorhiza incarnata ssp. cruenta (2n = 2x = 40) and its putative allotetraploid derivative D. lapponica (2n = 4x = 80). Photometric quantification of DNA content and morphology confirmed that triploids are abundant in sympatric populations in our study area. Allozyme segregation patterns in D. lapponica supported an allopolyploid origin, although unbalanced genotypes suggested rare pairings between homoeologous chromosomes. Photometric data and chromosome counts suggest backcrossing between the triploid hybrid and D. lapponica, and hence some hybrid fertility. Triploids are morphologically more similar to the tetraploids than the diploids, maybe owing to the hybrid origin of both triploids and tetraploids. The diploids and tetraploids were not more similar in the parapatric populations compared to when they occur in allopatry. This indicates that backcrossing rarely leads to introgression, or alternatively that allopatric populations are not isolated enough to prevent influx of pollen from the other species. Despite some evidence of backcrossing, our study gives few indications that widespread hybridization entails local breakdown of species boundaries. Rather, the hybrid zone may be a transient phenomenon due to intensive mowing, resulting in the opening of habitats and hence bringing the parental species into close contact.     

Spontaneous polyploids in marginal populations of Alopecurus bulbosus Gouan (Poaceae)

Natural populations of A bulbosus from three sites in southern England have been investigated and two were found to contain triploid and tetraploid plants in addition to normal diploids. Meiosis in the triploids was characterized by a high frequency of trivalent formation but, in the tetraploids, multivalents were formed at low frequencies and bivalents were the most common configurations. It is suggested that these are true autopolyploids that have probably arisen through somatic doubling since tetraploid cell lines were found in some anther loculi. The meiotic behaviour of the tetraploid plants is explained on the basis of bivalent-promoting mechanisms being present in the diploid progenitors.     

The role of tetraploids in the sexual-asexual cycle in dandelions (Taraxacum)

Apomictic plants often produce pollen that can function in crosses with related sexuals. Moreover, facultative apomicts can produce some sexual offspring. In dandelions, Taraxacum, a sexual-asexual cycle between diploid sexuals and triploid apomicts, has been described, based on experimental crosses and population genetic studies. Little is known about the actual hybridization processes in nature. We therefore studied the sexual-asexual cycle in a mixed dandelion population in the Netherlands. In this population, the frequencies of sexual diploids and triploids were 0.31 and 0.68, respectively. In addition, less than 1% tetraploids were detected. Diploids were strict sexuals, triploids were obligate apomicts, but tetraploids were most often only partly apomictic, lacking certain elements of apomixis. Tetraploid seed fertility in the field was significantly lower than that of apomictic triploids. Field-pollinated sexual diploids produced on average less than 2% polyploid offspring, implying that the effect of hybridization in the 2x-3x cycle in Taraxacum will be low. Until now, 2x-3x crosses were assumed to be the main pathway of new formation of triploid apomicts in the sexual-asexual cycle in Taraxacum. However, tetraploid pollen donors produced 28 times more triploid offspring in experimental crosses with diploid sexuals than triploid pollen donors. Rare tetraploids may therefore act as an important bridge in the formation of new triploid apomicts.     

Pathways to polyploidy: indications of a female triploid bridge in the alpine species <Emphasis Type="Italic">Ranunculus kuepferi</Emphasis> (Ranunculaceae)

Polyploidy is one of the most important evolutionary processes in plants. In natural populations, polyploids usually emerge from unreduced gametes which either fuse with reduced ones, resulting in triploid offspring (triploid bridge), or with other unreduced gametes, resulting in tetraploid embryos. The frequencies of these two pathways, and male versus female gamete contributions, however, are largely unexplored. Ranunculus kuepferi occurs with diploid, triploid and autotetraploid cytotypes in the Alps, whereby diploids are mostly sexual, while tetraploids are facultative apomicts. To test for the occurrence of polyploidization events by triploid bridge, we investigated 551 plants of natural populations via flow cytometric seed screening. We assessed ploidy shifts in the embryo to reconstruct female versus male gamete contributions to polyploid embryo and/or endosperm formation. Seed formation via unreduced egg cells (B_III hybrids) occurred in all three cytotypes, while only in one case both gametes were unreduced. Polyploids further formed seeds with reduced, unfertilized egg cells (polyhaploids and aneuploids). Pollen was highly variable in diameter, but only pollen >27 μm was viable, whereby diploids produced higher proportions of well-developed pollen. Pollen size was not informative for the formation of unreduced pollen. These results suggest that a female triploid bridge via unreduced egg cells is the major pathway toward polyploidization in R. kuepferi, maybe as a consequence of constraints of endosperm development. Triploids resulting from unreduced male gametes were not observed, which explains the lack of obligate sexual tetraploid individuals and populations. Unreduced egg cell formation in diploids represents the first step toward apomixis.     

The role of triploids in the origin and evolution of polyploids of <Emphasis Type="Italic">Turnera sidoides</Emphasis> complex (Passifloraceae,Turneroideae)

Triploids can play an important role in polyploid evolution. However, their frequent sterility is an obstacle for the origin and establishment of neotetraploids. Here we analyzed the microsporogenesis of triploids (x?=?7) and the crossability among cytotypes of Turnera sidoides, aiming to test the impact of triploids on the origin and demographic establishment of tetraploids in natural populations. Triploids of T. sidoides exhibit irregular meiotic behavior. The high frequency of monovalents and of trivalents with non-convergent orientations results in unbalanced and/or non-viable male gametes. In spite of abnormalities in chromosome pairing and unbalanced chromosome segregation, triploids are not completely sterile and yielded up to 67% of viable pollen. Triploids that originated by the fusion of 2n?×?n gametes of the same taxon showed more regular meiotic behavior and higher fertility than triploids from the contact zone of diploids and tetraploids or triploids of hybrid origin. The reproductive isolation of T. sidoides cytotypes of different ploidy level is not strict and the ‘triploid block’ may be overcome occasionally. Triploids of T. sidoides produce diploid and triploid progeny suggesting that new generations of polyploids could originate from crosses between triploids or from backcrosses with diploids. The capability of T. sidoides to multiply asexually by rhizomes, would enhance the likelihood that a low frequency of neopolyploids can be originated and maintained in natural populations of T. sidoides.     

Are tetraploids more successful? Floral signals,reproductive success and floral isolation in mixed-ploidy populations of a terrestrial orchid

Background and Aims Polyploidization, the doubling of chromosome sets, is common in angiosperms and has a range of evolutionary consequences. Newly formed polyploid lineages are reproductively isolated from their diploid progenitors due to triploid sterility, but also prone to extinction because compatible mating partners are rare. Models have suggested that assortative mating and increased reproductive fitness play a key role in the successful establishment and persistence of polyploids. However, little is known about these factors in natural mixed-ploidy populations. This study investigated floral traits that can affect pollinator attraction and efficiency, as well as reproductive success in diploid and tetraploid Gymnadenia conopsea (Orchidaceae) plants in two natural, mixed-ploidy populations.Methods Ploidy levels were determined using flow cytometry, and flowering phenology and herbivory were also assessed. Reproductive success was determined by counting fruits and viable seeds of marked plants. Pollinator-mediated floral isolation was measured using experimental arrays, with pollen flow tracked by means of staining pollinia with histological dye.Key Results Tetraploids had larger floral displays and different floral scent bouquets than diploids, but cytotypes differed only slightly in floral colour. Significant floral isolation was found between the two cytotypes. Flowering phenology of the two cytotypes greatly overlapped, and herbivory did not differ between cytotypes or was lower in tetraploids. In addition, tetraploids had higher reproductive success compared with diploids.Conclusions The results suggest that floral isolation and increased reproductive success of polyploids may help to explain their successful persistence in mixed-ploidy populations. These factors might even initiate transformation of populations from pure diploid to pure tetraploid.     

Sympatric occurrence of triploid, aneuploid and tetraploid weatherfish Misgurnus fossilis (Cypriniformes, Cobitidae)

Ploidy analyses of 116 weatherfish Misgurnus fossilis individuals revealed the sympatric occurrence of triploid, intermediate aneuploid and tetraploid specimens in a 1:1:4 ratio. No diploids were detected and the sex ratio of triploids and tetraploids was 1:1, while that of aneuploids was skewed at 3:1 for males. An origin of intermediate aneuploids from mating triploids with tetraploids is hypothesized.