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It is a notion particularly popular among Chinese high school Biology textbooks, that the red blood cells in frogs can undergo amitosis (not mitosis), a claim which I have not been able to find reference. A search on Pubmed led me to this article, which states:

The nucleus of the frog erythrocyte is not a regular ellipsoid; ... a single nuclear invagination located at one end of the long axis is frequently seen in the phase microscope.

However I have also been unable to find more recent researches on the nature of this observation, and whether it is related to amitosis. Can you help me find some?

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Unlike mammalian blood cells, blood cells in Amphibia are nucleated (as the paper in your link explains) and can perform cell division (here is an old paper from Acta Haematologica, 1954: https://www.karger.com/Article/Pdf/204534).

Your question is: is this division a regular mitosis or the so called amitosis? According to Barni et al., 1995, amphibian blood cells can perform amitotic cell division:

The proliferation of haemopoietic cells, detected by the anti-BrdU labelling index, was accompanied by absence of mitotic cell division and the appearance of cells showing features of amitosis (e.g. nuclear constrictions with bundles of electron-dense chromatin)

Source: Barni, S., Fraschini, A., Prosperi, E., Vaccaronel, R. and Bernini, F. (1995) ‘Possible occurrence of amitotic cell division during haemopoiesis in the Urodeles’, Comparative Haematology International, 5(3), pp. 183–188. doi: 10.1007/bf00368042.

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"Circulating" Red Blood cells are terminally differentiated cells, and cannot divide further. What appears to be the notion is that ejection of nuclear content is biased to one daughter cell (which is then eaten up by macrophages), and some researchers have dubbed it as amitotic process, instead of defining it as a maturation process. In case of humans the process of losing the nucleus from erythroblasts occurs prior to entry in circulation (in adults). You can read this if interested.

However, exceptions do exist... in newborns (and even in foetuses) nucleated red blood cells can be seen in circulation (although the count maybe low). In foetuses, this count is usually high when the oxygen demand increases (hypoxic/acidemia conditions). Read here.

The process from normoblasts to reticulocytes and finally RBCs is one of the setting points in evolution. In frogs (or amphibians in general), and other lower vertebrates (like fish, reptiles etc.) the presence of nucleated blood cells just identifies them as less advanced on the evolutionary-tree. Since higher order vertebrates have smaller blood vessels and complex respiratory organisation, evolution decided to squeeze out the nuclei from these cells to allow easy passage even via micron-thin vessels without compromising oxygen transport.

In short, the presence of nuclei doesn't necessarily determine the mitotic capability. RBCs have long been considered as terminally differentiated cells, and do not divide. If there are any studies reporting "circulating" RBCs as (a)mitotically active, I am unaware of it.

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