Algae World: images of meiosis in the diatom Amphora

diatom chromosome counts

It is extremely difficult to count chromosomes or examine chromosome morphology in diatoms, for two principal reasons: (1) the silica frustule prevents use of the normal 'squash preparation' methods, and (2) the diatom spindle is a narrow cylinder (during mitotic and meiotic divisions), around which the chromatin forms a dense mass at metaphase, in which individual chromosomes cannot usually be distinguished. The only stage at which chromosome counts can be made with any confidence in unsquashed material is during meiosis, at diakinesis. Even then, the chromosomes are spread out in three dimensions and can usually be counted only by tracing individual chromosomes through stacks of images.

Here we provide 'focusable' images of diakinetic or prometaphase meiotic nuclei of Amphora species. You can examine them almost as though you were looking down a microscope. By sliding a focus button or running a short video sequence, you can focus up and down through the specimens and count the chromosomes for yourself, to check details included in our papers. The photographs were taken using a Reichert Polyvar 2 photomicroscope at ×100 and with the ×1.25 intermediate lens, using a Polaroid DMC2 digital camera. This gives resolution of 265 pixels = 10 µm. Focusing was manual, at vertical intervals of c. 0.25 µm.

Further information and discussion can be found in Mann, D.G. & Poulíčková, A. (2010). Mating system, auxosporulation, species taxonomy and evidence for homoploid evolution in Amphora (Bacillariophyta). Phycologia 49: 183–201.

Amphora copulata: diakinesis

image stack with slider bar for focus. Note the particularly long bivalent at left-centre.
gif image
avi video file [we strongly recommend that you use QuickTime for this, because it will allow you to select individual frames or move backwards or forwards through the sequence]
images of one gametangium and the other gametangium (focused on the valves; scale bars = 10 µm)

Amphora ovalis: diakinesis

image stack with slider bar for focus. This shows a single diakinetic nucleus; our estimate is 14 or 15 bivalents.
avi video file (CAUTION! 4 Mb) In contrast to the stack in the slider bar version, the video clip shows both gametangia, which were at the same stage of meiosis. [we strongly recommend that you use QuickTime].

Amphora ovalis: diakinesis/prometaphase I

image stack with slider bar for focus. Metacentric and acro- or telocentric chromosome bivalents are visible
gif image
avi video file [we strongly recommend that you use QuickTime]

Amphora ovalis: diplotene/diakinesis

image stack with slider bar for focus. Metacentric and acro- or telocentric chromosome bivalents are visible
gif image (CAUTION! 3 Mb)
avi video file [we strongly recommend that you use QuickTime]

Amphora minutissima: diakinesis and following meiosis II (showing 4-nucleate stage with 2 nuclei degenerating)

The upper gametangium is in early diakinesis: the chromosomes have not separated as widely as in the A. copulata example. In the lower gametangium, which has recently completed telophase of meiosis II, four nuclei are visible. Two are compact and very densely staining, and have already begun to degenerate: they will remain visible as steadily shrinking but dark-staining 'pyknotic' nuclei until well after auxospore expansion has begun. The other two nuclei are more diffuse and are regaining the interphase condition of dispersed chromatin and heterochromatic granules.

image stack with slider bar for focus
gif image (CAUTION! 3.5 Mb)
avi video file [we strongly recommend that you use QuickTime]