The diatom Chaetoceros in ships’ ballast waters – survivorship of stowaways

Ship ballast water discharged by vessels into the receiving port is recognised today as an important vector for the spread of non-indigenous species and facilitates the introduction of potential invasive species. Here, we report on 18 species (of about 30 identified), both vegetative cells and spores, of the diatom genus Chaetoceros Ehrenberg found in ballast water collected from ships arriving at Canadian ports on theWest Coast (WC), East Coast (EC) and the Great Lakes (GL). We found live, vegetative Chaetoceros cells (one of the most abundant taxa) in 49% of the 57 ballast water samples. The highest density of viable spores enumerated in our counts was 414 cells L–1. In 62% of 52 samples examined using scanning electron microscopy (SEM), Chaetoceros spores were found, though fewer live, identifiable spores were found using light microscopy. Three reportedly harmful species, C. convolutus, C. danicus, C. debilis were encountered in WC samples, and additionally, C. cf. hispidus, a species not yet reported from Canada. C. ceratosporus and C. cf. subsecundus, to date reported only from the EC of the USA, now have been transported to the port of Vancouver, British Columbia. Our findings contribute to the assessment of the effectiveness of ballast water treatment via water exchange, and serve to evaluate the diversity of diatom vegetative cells and spores transported in ballast water tanks

Pax6 is required intrinsically by thalamic progenitors for the normal molecular patterning of thalamic neurons but not the growth and guidance of their axons

BACKGROUND: In mouse embryos, the Pax6 transcription factor is expressed in the progenitors of thalamic neurons but not in thalamic neurons themselves. Its null-mutation causes early mis-patterning of thalamic progenitors. It is known that thalamic neurons generated by Pax6(−/−) progenitors do not develop their normal connections with the cortex, but it is not clear why. We investigated the extent to which defects intrinsic to the thalamus are responsible. RESULTS: We first confirmed that, in constitutive Pax6(−/−) mutants, the axons of thalamic neurons fail to enter the telencephalon and, instead, many of them take an abnormal path to the hypothalamus, whose expression of Slits would normally repel them. We found that thalamic neurons show reduced expression of the Slit receptor Robo2 in Pax6(−/−) mutants, which might enhance the ability of their axons to enter the hypothalamus. Remarkably, however, in chimeras comprising a mixture of Pax6(−/−) and Pax6(+/+) cells, Pax6(−/−) thalamic neurons are able to generate axons that exit the diencephalon, take normal trajectories through the telencephalon and avoid the hypothalamus. This occurs despite abnormalities in their molecular patterning (they express Nkx2.2, unlike normal thalamic neurons) and their reduced expression of Robo2. In conditional mutants, acute deletion of Pax6 from the forebrain at the time when thalamic axons are starting to grow does not prevent the development of the thalamocortical tract, suggesting that earlier extra-thalamic patterning and /or morphological defects are the main cause of thalamocortical tract failure in Pax6(−/−) constitutive mutants. CONCLUSIONS: Our results indicate that Pax6 is required by thalamic progenitors for the normal molecular patterning of the thalamic neurons that they generate but thalamic neurons do not need normal Pax6-dependent patterning to become competent to grow axons that can be guided appropriately

Acid cleaned initial valves observed in brightfield LM.

<p>A & B—convex, relatively plain initial valves with single (A) and double (B) elevations, note band of quincunx pores at the valve margin; C through E—a strongly concentrically undulated valve at three different foci: C—focus on valve margin and striae; D—focus on peripheral part of the valve face, ribs and stout spines; E—focus near central elevation with smaller spines; F—inside view of initial valve with central elevation; G—outside view of a less convex initial valve, note ribbed margin.</p

Internal view of initial and post-auxospore valves as observed in SEM.

<p>A & B—note absence of rimoportula; C & D—rimoportulae near the valve mantle edge aligned perpendicularly to the mantle rim (arrowheads); E & F—hammock-shaped rimoportulae (arrowheads) with internal openings variably oriented with respect to the mantle rim; G & H—low-profile rimoportulae in typical position for a vegetative valve (arrowheads), underneath the mantle rim and parallel to rim’s circumference (focus is on the distal rim). Note imperfect striation and external ornamentation of the mantle on some of the valves.</p

Auxospore enlargement as observed in live material and brightfield microscopy.

<p>A—elongated cell represents auxospore development stage before it rounds up; B—auxospore with initially uneven lateral expansion; C—a small, almost spherical auxospore; D—near full size spherical auxospore filled up with chloroplasts; E—a nearly mature auxospore undergoing second partial plasmolysis, note small refractive structure present in parental theca, behind retracting initial cell protoplast; F—a mature auxospore with initial cell inside; G—a short filament of post auxospore cells with initial epi- and hypothecae incorporated into the viable end cells; H—expired auxospore, nearly clear of cell contents, showing an outline of initial epitheca (arrowheads); I-J—another deteriorating auxospore, nearly clear of cell contents at two different foci: I—focus on an initial epivalve, (arrowheads); J—focus on a large scale (arrowheads).</p

Representative EDS spectra.

<p>Spectra acquired from locations shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0141150#pone.0141150.g005" target="_blank">Fig 5A, 5E and 5I</a>, respectively for vestigial hypovalve, auxospore wall and incunabular scale (locations indicated by boxes). Note difference in vertical scale of the spectra in Fig 6B. Spectra from position 1 are taken from structure shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0141150#pone.0141150.g005" target="_blank">Fig 5</a> and spectra from position 2 from filter substrate which shows no detectable silicon peak. Gold peaks were generated from the conductive coating.</p

Auxospores and initial cells as observed in SEM; A-I water rinsed, and J-L acid-cleaned specimens.

<p>A—theca resulting from uneven division of the auxospore mother cell (compare to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0141150#pone.0141150.g004" target="_blank">Fig 4C</a>), note doughnut-shaped, lightly silicified rudimentary hypovalve (valve EDS spectrum shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0141150#pone.0141150.g006" target="_blank">Fig 6A-A1-2</a>); B—a young spherical auxospore with large scale (arrowheads) exposed (compare to<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0141150#pone.0141150.g003" target="_blank"> Fig 3I and 3J</a>); C—the same large scale showing detail of pitted ornamentation; D—partially collapsed, large auxospore probably captured during partial plasmolysis, note large diameter of parental theca demonstrating the second round of auxosporulation; E—a partially damaged auxospore with thick wall with siliceous elements (EDS spectrum shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0141150#pone.0141150.g006" target="_blank">Fig 6B-E1-2</a>), note that the small parent cell of this auxospore indicates first round of auxosporulation; F- spinose initial valve covered with thin and pliable remains of the auxospore wall; G—the same specimen as in F at greater magnification showing scaly incunabulae (arrowheads) in the auxospore wall; H—another initial frustule enshrouded in the auxospore wall demonstrating scaly incunabulae in the girdle region (arrowheads); I—individual incunabular scales (arrowheads) disassociated from the auxospore wall (EDS spectrum shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0141150#pone.0141150.g006" target="_blank">Fig 6C-I1-2</a>); J—complete initial frustule in girdle view, note epicingulum; K—external view of an initial valve; L—internal view of an initial valve.</p