Isotoma

Abstract: Complex hybridity in Isotoma petraea II Components and operation of a possible evolutionary mechanism

Abstract: Parts of windswept mountain ridges, covered with lichen heath, often remain snow-free during the winter. Due to freezing of their surroundings, collembolans and mites from the soil of this habitat are probably exposed to periods of oxygen deficiency during the winter. In the laboratory two species of oribatid mites from this habitat, Calyptozetes sarekensis (Trag.) and Carabodes labyrinthicus (Mich.) survived several weeks of anoxia at 0?C, some specimens for more than 96 d. Specimens of the collembolan Tetracanthella wahlgreni Linnaniemi survived similar periods, while Isotoma violacea Tullgren was killed within 6 to 8 d of anoxia. Survival of C. labyrinthicus was similar in specimens collected during the summer and winter, while T. wahigreni survived longer periods of anoxia during the winter. During anaerobiosis lactate was accumulated in C. labyrinthicus and T. wahigreni. It was also formed in I. violacea, which suggests that the formation of lactate in itself is not the only adaptation to survival under oxygen deficiency.

Abstract: The effects of collembolan grazing on arbuscular mycorrhizal (AM) fungi and plant growth were studied in a controlled experiment utilizing a mix of AM fungi and the dominant collembolan species (Isotoma sp.) indigenous to the experimental soil. Collembolan (+/− Col) effects were examined in the presence and absence of crop residue (+/− Litter) incorporated into the experimental soil. Significant interactions between collembolans and crop residue occurred for mycorrhizal colonization of roots and plant growth. In the absence of crop residue, collembolans reduced root length colonized by AM fungi, total plant dry mass and seed pod yield. However, in the presence of crop residue, collembolans had no effect on root colonization by AM fungi, and increased total plant mass and pod yield. Crop residue increased root colonization by AM fungi, numbers of bacteria and saprophytic fungi (colony forming units), small- ( 5 μm) diameter hyphal lengths in soil, and the final population of collembolans in soil. Collembolans reduced both small- and large-diameter hyphae in soil and the number of saprophytic fungi (colony forming units, p =0.052). Feeding preference experiments conducted in vitro showed that Isotoma sp. preferred to graze on mycorrhizal roots over nonmycorrhizal roots when given no other food choice. However, when crop residue was added as a food choice, Isotoma sp. showed a clear feeding preference for crop residue. We conclude that collembolan grazing on mycorrhizae can be detrimental to plant growth when other fungal food sources are limited, but grazing on mycorrhizal fungi does not occur when ample organic matter and associated saprophytic fungi are present in soils.

Abstract: The Lobelioideae is a cosmopolitan group whose cytoevolution is discussed on a model of primitively high diploid chromosome numbers, in which x = 14 is relatively plesiomorphic and x = 21 may be even more plesiomorphic. This model is suggested from the high frequency of lobelioid genera with x = 14, the probably plesiomorphic condition of x = 17 in the sister group Campanuloideae (Campanulaceae), and the primitive x = 15 in Stylidiaceae (Campanulales). It contrasts with that for a primitive x = 7 and paleopolyploidy to higher chromosome numbers. In our analysis, the genus Lobelia shows three broad cytoevolutionary groups, which probably have phylogenetic and infrageneric taxonomic significance: (1) woody diploids with x = 21 in Chile and woody diploids with x = 14 in Africa, Asia, and Hawaii; (2) herbaceous diploids with several series of dysploid chromosome numbers n = 19, 13, 12, 11, 10, 9, 8, 7, 6, mainly in Africa and Australia; (3) widespread and speciose herbaceous taxa based on a very derived n = 7, with recent frequent euploid rises (neopolyploidy) at or below the species level in subgenus Lobelia and allied or segregate genera. Other woody and herbaceous lobeliad genera have comparable cytoevolutionary patterns. New chromosome counts for Australian Lobelia, Pratia, and Isotoma illustrate the last two cytoevolutionary groups.