Is the sword moss (Bryoxiphium) a preglacial Tertiary relict?

Abstract

a b s t r a c tThe disjunction of floras between East Asia, Southeast North America, West North America, and Southwest Eurasia has been interpreted in terms of the fragmentation of a once continuous mixed mesophytic forest that occurred throughout the Northern Hemisphere due to the climatic and geological changes during the late Tertiary. The sword moss, Bryoxiphium, exhibits a distribution that strikingly resembles that of the mesophytic forest elements such as Liriodendron and is considered as the only living member of an early Tertiary flora in Iceland. These hypotheses are tested here using molecular dating analyses and ancestral area estimations. The results suggest that the extant range of Bryoxiphium results from the fragmentation of a formerly wider range encompassing North America and Southeast Asia about 10 million years ago. The split of continental ancestral populations is too recent to match with a continental drift scenario but is spatially and temporally remarkably congruent with that observed in Tertiary angiosperm relict species. The timing of the colonization of Iceland from Macaronesian ancestors, about two million years ago, is, however, incompatible with the hypothesis that Bryoxiphium is the only living member of an early Tertiary flora of the island. Alaska was recurrently colonized from East Asia. The ability of Bryoxiphium to overcome large oceanic barriers is further evidenced by its occurrence on remote oceanic archipelagos. In particular, Madeira was colonized twice independently from American and East Asian ancestors, respectively. The striking range disjunction of Bryoxiphium is interpreted in terms of its mating system, as the taxon exhibits a very singular pattern of spatial segregation of the sexes.

On the Bryoxiphium norvegicum and B. japonicum (Bryoxiphiaceae, Bryopsida).

“…The previous phylogeographic analysis of the genus based on chloroplastic DNA markers (Patiño et al, 2016) presented a picture only partly congruent with our study results. Their analysis indicated a previous, wider distribution of Bryoxiphium, strongly fragmented now, and its ability to long-distance dispersal.…”

“…Accepting this scenario, the East Asian origin of modern populations for the Holarctic range of the species becomes obvious. However, the minimal difference between Iceland and Baikal populations (10 000 km from each other, opposite parts of the globe in Northern Hemisphere) poses necessarily a difficulty of explanation: (1) either the DNA structure of this haplotype is so conservative that allows no afteward variation, or, (2) both of these populations are the result of very recent spreading, and the time of divergence between North American and East Asian populations evaluated by Patiño et al (2016) as about 10 million years is an overestimation; or (3) hybridization events in East Asia took place so fast that they resulted in a lack of variation of ITS sequences in Bryoxiphium, which looks like an outstanding phenomenon. Naturally, a combination of these explanations and other possible ones cannot be excluded.…”

“…Á. Löve & D. Löve in East Asia. A strongly disjunct distribution of the genus was in the focus of a biogeographical discussion of Steere (1937), Löve & Löve (1953) and Patiño et al (2016). Steere (1937) noted that the material from Japan and Korea called B. japonicum is probably very close and perhaps indistinguishable from B. norvegicum.…”

“…japonicum nested in clade of B. norvegicum based on rbcL sequences. Recently Patiño et al (2016) studied distribution patterns of the genus using four cpD-NA loci from 39 specimens of Bryoxiphium, covering almost the entire range of the genus. They found that East Asian Bryoxiphium comprises three groups of haplotypes, one being sister to all other specimens.…”

Future directions and priorities for Arctic bryophyte research

“…The small stature of bryophytes allows them to inhabit microhabitats and may have facilitated their survival in unglaciated microhabitats within otherwise glaciated areas (Anderson 1963; recently discussed by Patiño et al 2016). Alternatively, bryophytes may be cryopreserved during glaciation with regeneration following glacial retreat, as recently observed, albeit over shorter periods of centuries to millennia, for Arctic and Antarctic mosses (Fig.…”