Thomas Schröder

Publications (12) View all

• Article: Intraclonal variation in propensity for mixis in several rotifers: variation among females and with maternal age

  John J. Gilbert, Thomas Schröder
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  ABSTRACT: In 14 previously published experiments with clones of three species of Brachionus, Epiphanes ukera, and Rhinoglena frontalis, females cultured singly in small volumes from birth through most of their reproductive period usually produced some mictic daughters. Here, these data are analyzed to test two hypotheses: the propensity of females in a clone to produce mictic daughters will vary significantly among females, and will decrease with female age. Significant heterogeneity (χ2) among replicate females was demonstrated in two clones of B. calyciflorus from Florida, in single clones of this species from Georgia, Texas, Spain, and Australia, in a clone of B. angularis from Argentina, and in single clones of E. ukera and R. frontalis from Germany. No significant heterogeneity was found in two other clones of B. calyciflorus from Florida and in a clone of B. variabilis from Spain. Significant heterogeneity among females of a clone could be caused by chance fluctuations during development, differences in birth order, or epimutations. This heterogeneity is an important component of a bet-hedging strategy to balance production of diapausing fertilized eggs and future population growth by female parthenogenesis. The propensity of a female to produce mictic daughters did not decrease with her age. In experiments with six different clones from four strains of B. calyciflorus, and with single clones of E. ukera and R. frontalis, only one clone of B. calyciflorus showed significant variation (ANOVA) in the proportion of mictic daughters produced in three or four successive one- or two-day periods. In this clone the proportion of mictic daughters increased with maternal age.
  Hydrobiologia 04/2012; 593(1):121-128. · 1.78 Impact Factor
• Source

  Available from: Thomas Schröder

  Article: Cryptic speciation in the cosmopolitan Epiphanes senta complex (Monogononta, Rotifera) with the description of new species

  Thomas Schröder, Elizabeth J. Walsh
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  ABSTRACT: Many rotifers are considered to be cosmopolitan species including Epiphanes senta, which has been reported from all continents including Antarctica. To determine the potential for cryptic speciation in this species, we examined morphology, mating behavior and genetic divergence in three populations: (1) a tropical alpine lake on Mauna Kea (HI, USA), (2) temporary rock pools of the Chihuahuan Desert (TX, USA), and (3) floodplain habitats of the Oder River (Germany). Morphological differences among these populations are restricted to the surface structure of diapausing eggs and the shape of the rami. In crossmating experiments, German males never fertilized females from the Texan population. Texan males mated with hatching females from the German population. However, juvenile mortality among these fertilized females reached 40% and was significantly higher than juvenile mortality after intrapopulational mating (2%). Surviving females did not produce viable diapausing eggs. Hawaiian males readily mated with newborn Texan females, which produced viable offspring. They also copulated with females from the German population, but these females never produced diapausing eggs. Genetic distance among the three populations was considerable, ranging from 10.9 to 12.5% for 648bp of the mitochondrial cox1 gene. Morphological, reproductive, and genetic differentiation among the studied populations demonstrates E. senta is actually a species complex. Three new species within the E. senta species complex are described.
  Hydrobiologia 04/2012; 593(1):129-140. · 1.78 Impact Factor
• Article: Genetic differentiation, behavioural reproductive isolation and mixis cues in three sibling species of monogonont rotifers.

  Thomas Schröder, Elizabeth J Walsh
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  ABSTRACT: Many aquatic species usually considered to be 'cosmopolitan' have been identified as cryptic species complexes, based on deep genetic differentiation. However, reproductive isolation among sibling cryptic species has rarely been studied, and interspecific hybridization is common in some taxa.We investigated isolation mechanisms and possible introgression among three cyclical parthenogenetic rotifer species in the Epiphanes senta complex that are found in very different freshwater habitats: temperate floodplains, subtropical desert rock pools and a tropical alpine lake. Whereas Epiphanes ukera is reproductively isolated from E. chihuahuaensis and E. hawaiiensis, the latter hybridize under laboratory conditions.While reproductive isolation is incomplete, RAPD profiles indicated unique genetic signatures and showed no evidence for introgression, indicating that these three species are diverging and have independent evolutionary trajectories.Testing cues for sexual reproduction in these cyclic parthenogens demonstrated that mixis in E. chihuahuaensis and E. ukera is influenced by population density, whereas E. hawaiiensis females rarely produce mictic offspring regardless of density. Different mixis cues are likely to separate sexual periods and effectively cause reproductive isolation between the species. Epiphanes ukera and E. chihuahuaensis males display mate guarding behaviour, and E. ukera males distinguish between conspecific and heterospecific females in mate choice experiments. Geographic isolation, along with different cues for mixis induction and mate recognition, act as reproductive barriers among these sibling species.
  Freshwater Biology 12/2010; 55(12):2570-2584. · 3.29 Impact Factor
• Article: Maternal age and spine development in the rotifer Brachionus calyciflorus: increase of spine length with birth orders1

  THOMAS SCHRÖDER, JOHN J. GILBERT
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  ABSTRACT: Summary1. Maternal effects have long been known to influence phenotypic plasticity in rotifers. Females in Brachionus calyciflorus and several other species produce long-spined offspring when the predatory rotifer Asplanchna is present; B. calyciflorus also develops short spines when food concentrations are low. These spines protect against predation and decrease food threshold concentrations.2. Some strains of B. calyciflorus develop long spines even in the absence of Asplanchna and other environmental stimuli. We demonstrate in this study that spine length in such cases is dependent on the age of the mother.3. In strains from Florida and Georgia, offspring spine length increased significantly with birth order, sometimes to lengths formerly observed only in the presence of Asplanchna. Significant variation in this trait was found among and within clones of a strain. Offspring body size also increased with maternal age. This is the first time maternal age has been shown to affect rotifer morphology.4. These birth-order effects may have important ecological implications and explain phenotypic plasticity and polymorphism in body size and spine length in populations when predators are absent and food concentrations are high. They may be a bet-hedging mechanism to assure adaptation to rapid changes in predation pressure or food conditions.
  Freshwater Biology 04/2009; 54(5):1054 - 1065. · 3.29 Impact Factor
• Source

  Available from: Roberto Rico-Martínez

  Article: Rotifers from selected inland saline waters in the Chihuahuan Desert of México.

  Elizabeth J Walsh, Thomas Schröder, Robert L Wallace, Judith V Ríos-Arana, Roberto Rico-Martínez
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  ABSTRACT: In spite of considerable efforts over past decades we still know relatively little regarding the biogeography of rotifers of inland waters in México. To help rectify this we undertook an extensive survey of the rotifer fauna of 48 water bodies in the Chihuahuan Desert of México. Of the sites surveyed, 21 had salinities >or= 2000 microS cm-1 and in these we found 57 species of monogonont rotifers and several bdelloids. Species richness in the saline sites varied widely, with a range in species richness of 1 to 27 and a mean (+/- 1SD) = 8.8 (+/- 6.2). Collectively all sites possess relatively high percent single- and doubletons, 33.3 and 21.7%, respectively. Simpson's Asymmetric Index indicated that similarity in rotifer species composition varied widely among a set of 10 sites. These were selected because they were sampled more frequently or represent unusual habitats. These SAI values ranged from 0.00 (complete dissimilarity) to 1.00 (complete similarity). The Jaccard Index varied between 0.00 and 0.35. This observation probably reflects similarities and differences in water chemistry among these sites. Inland saline systems differed in their chemical composition by region. Conductivity was related to hardness and alkalinity. In addition, hardness was positively associated with chloride and sulfate. RDA showed that several species were positively associated with chloride concentration. Other factors that were significantly associated with rotifer species included the presence of macrophytes, nitrate content, oxygen concentration, TDS, latitude and whether the habitat was a large lake or reservoir. This study illustrates the diversity of the rotiferan fauna of inland saline systems and the uniqueness among waterbodies. Conservation of these systems is needed to preserve these unique sources of biodiversity that include rotifers and the other endemic species found in association with them.
  Saline Systems 02/2008; 4:7. · 1.18 Impact Factor