The allogamous raphid diatom Achnanthes longipes C. A. Agardh possesses a complex breeding system involving interactions between three types of clone: monoecious, unisexual and bisexual. Previous studies showed that these three types can be crossed with each other, with a tendency for sexual characteristics to be inherited: inbred monoecious lineages gave rise to monoecious or, very rarely, to bisexual clones, while inbred unisexual lineages yielded unisexual and bisexual clones. The current paper reports on the progeny of crosses between monoecious and unisexual clones and their inbred offspring. All three types of clone appeared in the F1 and F2, although unisexual clones of opposite sex to the parental clone were not found. Inbreeding depression was observed and also a tendency for ‘normal’ auxosporulation (producing two auxospores per pair of gametangia) to be replaced by ‘reduced’ or ‘intermediate’ auxosporulation (producing one auxospore per pair). In addition, patterns of incompatibility were observed that were not seen during earlier studies of clones isolated directly from nature. These included the inability of some F1 clones to mate with each other, in spite of compatibility with all other clones examined (unisexual, bisexual and monoecious).

This paper continues a series of studies of the allogamous raphid diatom Achnanthes longipes, which has a complex reproductive system combining unisexual, bisexual and monoecious behaviour. Following earlier work on the effects of inbreeding in the progeny of crosses between two unisexual clones, we studied the progeny of clones that are capable of a high degree of selfing in monoclonal culture (‘monoecious clones’). Three generations of selfed progeny were examined. In addition, we investigated the F1 generation obtained after crossing two different monoecious clones. Monoecious clones produced monoecious or, more rarely, bisexual progeny, but did not give rise to unisexual progeny. As in inbred lineages made by crossing closely related unisexual clones, inbreeding in monoecious lineages leads to a reduction in the number of gametes formed by the gametangia, from two to one. Inbred clones exhibit marked inbreeding depression and only three inbred generations were possible in monoecious lineages. In the third, final inbred generation, monoecious sexual reproduction was initiated in monoclonal cultures but gametes rarely fused and none of the auxospores and initial cells that were formed were viable; this also occurred when the inbred clones were crossed with any of the other clones available. The significance of inbreeding and other aspects of the breeding system in A. longipes is discussed.

Monoecious, bisexual and unisexual clones of Achnanthes longipes were isolated from the Black Sea and studied in laboratory culture. Clones differed in their growth characteristics: in monoecious clones the cells formed tufted aggregations while in other clones they were more dispersed. Bisexual and unisexual clones exhibited intraclonal (monoecious) reproduction, but only at a very low frequency and usually within a more restricted size range than in monoecious clones. Interclonal crosses were made in all possible pairwise combinations. Abundant auxosporulation took place in all crosses, except where unisexual clones of the same sex were incubated together. Auxosporulation was more vigorous and occurred over a wider size range in interclonal crosses than during monoecious reproduction. Sexual reproduction is isogamous. In the commonest pattern of auxosporulation, two paired gametangia each produce two gametes, which fuse to give two auxospores. More rarely (9% of pairs), the gametangia produce only one gamete apiece, and hence only one auxospore. In addition, very small cells can enlarge vegetatively, although genetic or cytological damage sometimes compromises their long-term viability.

Achanthes inflata (Kütz) Grun. et la variété elata (Leud.-Fontm.) Hust. ont été étudiés au microscope électronique. L'espèce et la variété, outre la forme de la valve, se différencient par quelques caractères structuraux. Une attention particulière a été portée sur l'organisation du cingulum ; le nombre d'éléments qui le composent est variable en fonction du stade où se trouve la cellule entre deux divisions.

Raphe-like vestiges have been found regularly in two new taxa which presumably belong to Fragilaria, one of them livingin several lakes of the Kerguelen Islands, the other in the Yellowstone National Park, Wyoming, U.S.A. The latter onepossesses septa-like elements as integrated parts of a girdle band. Other septa-like elements can occur in different speciesof Fragilaria. They represent appendices of the valvocopula, located beneath the transapical costae. Structures like linkingspines on marginal parts of the valve face occur in an Achnanthes species. Most of these characters seem to havebeen unknown in these taxa until recently.