Abstract Detail

Paleobotany

Tempo and mode in the evolution of pith – the contribution of Early Devonian (c. 400 Ma old) euphyllophytes of the Battery Point Formation (Gaspé Peninsula, Quebec, Canada).

The stems of most extant euphyllophytes possess vascular architectures characterized by the presence of a central parenchymatous pith: eusteles or siphonosteles. However, the fossil record demonstrates that the earliest euphyllophytes (Early Devonian) had protosteles that lacked a pith, and that steles with central parenchyma arose later, in the Middle Devonian. Not surprisingly, the evolutionary origin of the pith has elicited debate since the beginnings of plant morphology. By the early decades of the 20^th century, most morphologists agreed on an intrastelar origin of the pith, i.e., by changes at the center of a protostele. Early hypotheses proposed to explain it – increase in stele size and changes in the developmental fate of cells in the central area of the stele – had implicit yet unequivocal evo-devo tones. In subsequent refinements if these ideas, a transformational series based on Permian osmundaceous ferns was used to propose that the developmental changes involved increase in diameter, shortening, and progressive parenchymatization of central metaxylem tracheids. Likewise, a model associating xylem cell fates to auxin gradients determined by apical meristem size and lateral appendage density suggested that origin of the pith was due to increases in both of these shoot features. Two early euphyllophytes from the Emsian (Early Devonian, c. 400 Ma) Battery Point Formation of the Gaspé Peninsula (Quebec, Canada) now add data relevant to the evolution of pith. One of them, a previously undescribed plant of putative cladoxylopsid affinites, possesses a core of parenchyma at the center of actinostelic axes, thus marking the oldest evidence for pith in the euphyllophyte clade and demonstrating that evolution of the pith was well underway in the Early Devonian. The other one, described recently as Leptocentroxyla tetrarcha, features actinostelic xylem whose anatomy – protoxylem architecture, tracheid wall thickenings and patterns of variation in tracheid size – supports a hypothesis of developmental changes that can explain the evolution of pith. Tracheids at the center of xylem in Leptocentroxyla exhibit a simplified form of the Psilophyton-type thickenings that characterize the typical metaxylem of the plant. These tracheids, interpreted as paedomorphic metaxylem, are similar in size to the typical metaxylem, in the main axis, and are positionally equivalent to protoxylem in the lateral appendages. The positioning and inferred developmental attributes of the paedomorphic metaxylem support evolution of the pith by delayed and shortened differentiation of protoxylem. This hypothesis posits (1) delay in onset of differentiation and lengthening of cell growth duration in a central protoxylem strand; and (2) shortening of the interval of differentiation of those tracheids, leading to progressive simplification (and eventual loss) of secondary wall thickenings, and replacement of tracheids with a central parenchymatous area. NAC-domain transcription factors and their interactions with abscisic acid may have provided the regulatory substrate for the developmental changes that led to the evolution of pith. These changes could have been orchestrated by selective pressures associated with the expansion of early vascular plants, during the Early Devonian, into water-stressed upland environments.

1 - California State Polytechnic University - Humboldt, Department of Biological Sciences, Arcata, California, 95521, USA

Keywords:
fossil
paleobotany
Evolution
Development
Evo-Devo
xylem
protoxylem
tracheid
paedomorphic
Anatomy
pith
Devonian
euphyllophyte
stele
actinostele
protostele.

Presentation Type: Oral Paper
Number: PB4001
Abstract ID:472
Candidate for Awards:None