Homospory 2002: An Odyssey of Progress in Pteridophyte Genetics and Evolutionary Biology

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Symposium on scientific literacy: Introduction

This is the publisher's version, also available electronically from http://www.amjbot.org.Not applicabl

C3 Photosynthesis in the Gametophyte of the Epiphytic CAM Fern Pyrrosia Longifolia (Polypodiaceae)

This is the publisher's official version, also available at: http://www.jstor.org/stable/2445689Sporophytes of some epiphytic species in the fern genus Pyrrosia exhibit Crassulacean acid metabolism (CAM), generally considered to be a derived physiological response to xeric habitats. Because these species alternate between independent sporophytic and gametophytic generations yet only the sporophyte has been characterized physiologically, experiments were conducted to determine the photosynthetic pathways present in mature sporophytes, immature sporophytes, and gametophytes of Pyrrosia longifolia. Diurnal C 0 2 exchange and malic acid fluctuations demonstrated that although the mature sporophytes exhibited CAM, only C3 photosynthesis occurred in the gametophytes and young sporophytes. Consideration of the above results and those from previous studies, as well as the life cycle of ferns, indicates that the induction of CAM probably occurs at a certain developmental stage of the sporophyte and/or following exposure to stress. Elucidation of the precise mechanisms underlying this C^-CAM transition awaits further research

Patterns of Genetic Variation in Southern Appalachian Populations of Athyrium filix‐femina var. asplenioides (Dryopteridaceae)

Allozyme variation (17 loci coding 11 enzymes) was investigated in 14 populations of the fern Athyrium filix‐femina var. asplenioides arrayed at differing elevations and latitudes in the southern Appalachians. Allozyme fingerprints showed that asplenioides individuals comprise meandering, overlapping clones usually ≤3 m in extent, occasionally forming larger clones of up to 17 m. Levels of genetic variability in populations (means: , , ) were near the averages for both ferns and seed plants. General conformance to Hardy‐Weinberg expectations indicated a predominantly outcrossing mating system. Hierarchical F statistic analysis and occasional deficits and excesses of heterozygotes indicated population substructure. Similar allele frequencies across all populations resulted in low to moderate values (mean ; ) and high values of genetic similarity (mean ; mean ). Hierarchical analysis indicated that neither regional proximity ( ) nor elevation ( ) contributed substantially to divergence among populations ( ), a result corroborated by UPGMA analysis that clustered together populations from different regions and of different elevational class. Southern Appalachian asplenioides differed from more eastern asplenioides populations of the piedmont and coastal plain in having higher frequencies of Pgm‐2c and Tpi‐2B, alleles characteristic of the more northern variety angustum. Nonetheless, genetic distinctness of the two varieties was maintained. We hypothesize that higher frequencies of angustum alleles in the southern Appalachian asplenioides populations are the result of introgression from angustum that persisted at high elevations as both taxa migrated northward following the retreat of the Wisconsinan glacier

Phylogenetic Relationships of the Enigmatic Malesian Fern Thylacopteris (Polypodiaceae, Polypodiidae)

Thylacopteris is the sister to a diverse clade of polygrammoid ferns that occurs mainly in Southeast Asia and Malesia. The phylogenetic relationships are inferred from DNA sequences of three chloroplast genome regions (rbcL, rps4, rps4‐trnS IGS) for 62 taxa and a fourth cpDNA sequence (trnL‐trnF IGS) for 35 taxa. The results refute previously proposed close relationships to Polypodium s.s. but support suggested relationships to the Southeast Asiatic genus Goniophlebium. In all phylogenetic reconstructions based on more than one cpDNA region, we recovered Thylacopteris as sister to a clade in which Goniophlebium is in turn sister to several lineages, including the genera Lecanopteris, Lepisorus, Microsorum, and their relatives. Goniophlebium and allies comprise a significant component of vascular fern epiphytes in the rain forests of Southeast Asia and Malesia. The relationships of the genus Thylacopteris as at the base of the clade comprising the genera Goniophlebium, Lecanopteris, Lepisorus, Microsorum, and their relatives indicate that this entire lineage arose in Malesia and subsequently dispersed to continental Asia, Australia, the Pacific, and Africa

A Total Evidence Approach to Understanding Phylogenetic Relationships and Ecological Diversity in Selanginella subg. Tetragonostachys

• Premise of the Study: Several members of Selaginella are renowned for their ability to survive extreme drought and “resurrect” when conditions improve. Many of these belong to subgenus Tetragonostachys , a group of ~45 species primarily found in North and Central America, with substantial diversity in the Sonoran and Chihuahuan Deserts. We evaluated the monophyly and the age of subgenus Tetragonostachys and assess how drought tolerance contributed to the evolution of this clade. • Methods: Our study included most Tetragonostachys species, using plastid and nuclear sequences, fossil and herbarium records, and climate variables to describe the species diversity, phylogenetic relationships, divergence times, and climatic niche evolution in the subgenus. • Key Results: We found that subgenus Tetragonostachys forms a monophyletic group sister to Selaginella lepidophylla and may have diverged from other Selaginella because of a Gondwanan–Laurasian vicariance event ca. 240 mya. The North American radiation of Tetragonostachys appears to be much more recent and to have occurred during the Early Cretaceous–late Paleocene interval. We identifi ed two signifi cant and nested ecological niche shifts during the evolution of Tetragonostachys associated with extreme drought tolerance and a more recent shift to cold climates. Our analyses suggest that drought tolerance evolved in the warm deserts of southwest North America and may have been advantageous for colonization of cold and dry boreal climates. • Conclusions: Our investigation provides a foundation for future research addressing the genomics of ecological niche evolution and the potential role of reticulate evolution in Selaginella subgenus Tetragonostachys .The authors thank P. Korall for sharing data. S. Buerki, N. Alvarez, and B. Marazzi provided technical assistance and valuable comments on the manuscript. N.A. was funded by the Swiss National Science Foundation (grant no. PBNEP3-132747). C.L.A. was funded by the Swedish Research Council

Sex and the Single Gametophyte: Revising the Homosporous Vascular Plant Life Cycle in Light of Contemporary Research

Homosporous vascular plants are typically depicted as extreme inbreeders, with bisexual gametophytes that produce strictly homozygous sporophytes. This view is promulgated in textbook life cycles despite ample evidence that natural populations of most species regularly outcross. We review research on a variety of mechanisms, including genetic load, asynchronous production of eggs and sperm, and pheromonal control of gamete production, that actively promote heterozygosity in ferns and lycophytes. Evolution of the land plants cannot be reconstructed without accurate depictions of the unique life cycle that has helped make ferns the second most diverse lineage of vascular plants on Earth. With revised illustrations and definitions, we provide scientists, educators, and students with a contemporary understanding of fern and lycophyte reproduction, revealing them as evolutionarily dynamic and exploiting a wide range of mating systems

Species Concepts in Pteridophytes: Summary and Synthesis

Volume: 79Start Page: 90End Page: 9

RbcL Sequences Provide Phylogenetic Insights among Sister Species of the Fern Genus Polypodium

Volume: 85Start Page: 361End Page: 37