Droop, S.J.M., Mann, D.G. & Lokhorst, G.M. (2000). Spatial and temporal stability of demes in Diploneis smithii / D. fusca (Bacillariophyta) supports a narrow species concept. Phycologia, 39, 527-546 (published 5 March 2001).

An earlier study demonstrated the presence of 11 morphologically distinct demes ('morphotypes') within the Diploneis smithii / D. fusca species complex in a single sample from Ganavan near Oban on Scotland's west coast. Two of these demes were subsequently shown to occur elsewhere around the British coastline and to keep their identity wherever they occurred together. However, the conclusion that the demes were sufficiently distinct and widespread for them to be described as separate species has been criticized on the grounds that no evidence exists that there are reproductive barriers between them which could provide independent evidence for their relationships. It has not been possible, thus far, to grow the species in culture (a prerequisite for controlled breeding experiments), and so independent evidence has been sought elsewhere. It is shown here that nine of the 11 demes from Ganavan also occur in Cumbrae (in the Firth of Clyde, some 80 km SSE of Oban), not only in recent samples but also in a sample collected in 1858. A detailed morphometric and ultrastructural analysis shows that the demes are stable over 80 km and 140 years. This is discussed in the context of the length of diatom life cycles, and it is concluded that at least several generations of size reduction and restitution must have occurred since 1858. In most diatoms, size reduction and restitution accompany allogamous sexual reproduction, so it is inferred that the demes within the Diploneis smithii / D. fusca complex are probably true sexual species.