Spatial patterns of photobiont diversity in some Nostoc‐containing lichens

Abstract

Patterns of photobiont diversity were examined in some Nostoc-containing lichens using the nucleotide sequence of the cyanobacterial tRNA Leu (UAA) intron. Lichen specimens collected in northwestern USA were analysed and the sequence data were compared with tRNA Leu (UAA) intron sequences previously obtained from lichens in northern Europe. Generally, it is the species identity of a lichen rather than the geographical origin of the specimen that determines the identity of the cyanobiont. Identical intron sequences were found in Peltigera membranacea specimens collected in Oregon (USA) and in Sweden, and very similar sequences were also found in Nephroma resupinatum thalli collected in Oregon and Finland. Furthermore, in mixed assemblages where two Peltigera species grew in physical contact with each other, the different lichen species housed different photobiont strains. There is however not a one-to-one relation between mycobiont and photobiont as some intron sequences were found in more than one lichen species, and different intron sequences were found in different samples of some lichen taxa. Peltigera venosa exhibited a higher level of photobiont diversity than any other lichen species studied, and several intron sequences could for the first time be obtained from a single thallus. It is not clear whether this is evidence of lower cyanobiont specificity, or reflects an ability to exhibit different degrees of lichenization with different Nostoc strains. In one specimen of P. venosa, which contained bipartite cyanosymbiodemes and tripartite, cephalodiate thalli, both thallus types contained the same intron sequence.

Assessing host specialization in symbiotic cyanobacteria associated with four closely related species of the lichen fungusPeltigera

“…Sharing of photobiont genotypes among different host species has also been reported previously among members of the Nephromataceae (Rikkinen et al, 2002;Lohtander et al, 2003;Wirtz et al, 2003), as well as within bryophytes (Costa et al, 2001) and cycads (Costa et al, 1999), but ours is the first study to include large enough sample sizes of both plant symbionts and lichen photobionts to infer that symbiont sharing among unrelated hosts is common. These results contradict earlier studies, based on smaller datasets, which suggested that species-level host specialization was prevalent in cyanolichens and that host species was a better predictor of symbiont genotype than geography (Paulsrud et al, , 2000. Wirtz et al (2003) suggested that the lack of host specialization observed in Antarctic cyanolichens might be due to selection pressure for generalism in harsh environments, but our data indicate that low host specialization can be found also in temperate cyanolichens.…”

Form species Nostoc commune (Cyanobacteria).

“…However, intron WH12\1880, from N. commune collected from Surabaya, Java (118 years previously and more than 15 000 km distant), is 100 % identical to ENG\1996 and BBC\1990. The significance of very small differences in sequence similarity is not clear, although Paulsrud et al (2000) were able to use small differences in the variable I regions of tRNA Leu (UAA) introns to compare geographical distribution patterns of Nostoc cyanobionts.…”

“…It was suggested that the two classes of intron may correlate with different physiological capacities of the cyanobionts, although there were insufficient differences within the conserved regions of the intron to support or reject this proposal. In a recent study, these same authors used tRNA Leu (UAA) intron analysis to study geographical patterns of diversity in Nostoc-containing lichens (Paulsrud et al, 2000).…”

Tansley Review No. 116

“…Lack of diversity within a thallus is consistent with a single event of cyanobiont acquisition during the formation of a bipartite thallus and the formation of new cephalodia by cyanobiont acquisition from older ones. However, the possible occurrence of different Nostoc strains in differrent cephalodia of a single Peltigera venosa thallus (Paulsrud et al, 2000) suggests that, at least in some lichens, the formation of new cephalodia can also involve fresh cyanobiont acquisition from the environment.…”