Evolution of gametophytes molphologies of Pteridophytes

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Gametophytes. of Pteridophytes show wide variety of forms, i.e. tuberous, cordate-thalloid, strap-like, ribbon-like, and filamentous shapes. These gametophyte forms have been developed in relation to Pteridophyte evolutions. The tuberous gametophytes are subterranean with endophytic fungi, and found in lycophytes, and Botrychium and Psilotum, the latter two of which are plant groups of early branch of ferns. Furthermore Osmunda, the first branch of the leptosporangate ferns, has been known to have massive gametophytes at early stage of development. The gametophytes of the rest of ferns are mainly cordate, and rarely ribbon-like or irregular in shape. These facts suggest that massive tuberous forms are primitive and cordate formes are derived from tuberous forms, and then ribbon-like or irregular shaped gametophytes are evolved in advanced groups such as Polypodiaceae and Vittariaceae. We compared gametophyte developments in many fern taxa using the sequential-observation technique. Tuberous gametophytes of Botrychiaceae are formed due to cell divisions in random orientation after spore germination, and the apical meristem with three-sided apical cell are differentiated later. In Osmundaceae, the massive part is only limited to posterior part of the early gametophyte, and two-sided apical cell is formed in anterior portion one cell layer thick. Due to activity of the apical cell, cordate part is added to the massive part. In cordate gametophytes of other ferns, early massive portions are totally lacking. It is concluded that shift of shape from the massive to the cordate has been conducted by lacking of massive portion, and additional gain of the two-sided apical cell. In ribbon-like gametophytes, there is an apical cell, but the activity of the apical meristem is weak compared to the marginal meristem, which is next to the former. In irregular-shaped gametophytes, the apical meristem disappears at early developmental stage, and is replaced by the marginal meristem.