Multidisciplinary studies of the diversity and evolution in river-weeds

Abstract

The moss-like river-weeds or Podostemaceae offer a special opportunity to study the diversity and evolution of plants that are adapted to extreme environments. This paper reviews multidisciplinary studies on this subject. Based on field work in the four continents, we discovered many species and several genera that are new components of biodiversity, and revealed the Podostemaceae floras of East Asia, Southeast Asia, and Australia. The historical biogeography of the family, i.e., the change in distribution in space and time, is characterized by a few dispersals between continents, followed by diversification within each continent. Local species may be derived from parts of separated populations of parental species, which consequently are paraphyletic. The remarkable morphological adaptations of Podostemaceae include the development of the horizontal axis in plant body, with which the plants adhere to rock surfaces under violent current. The vertical axis is reduced or lost and the horizontal axis develops in the embryo and seedling. We also found saltational organ-level variation, such as presence or absence of shoot, shoot apical meristem, root, and root cap; the form of shoot and root; the mode of root branching and leaf production; and the number of cotyledons. Morphological evolution may not be always adaptive to the habitats, which are rocks periodically submerged across the distribution range. Analyses of shoot regulatory gene expression found that, in contrast to the expression pattern in primitive species with ordinary shoots, which is comparable with Arabidopsis, the unique pattern in derived species may result in 'fuzzy' morphology of the shoot and leaf. Finally, problems for future study are pointed out.

Paracladopus chiangmaiensis (Podostemaceae), a new generic record for China and its complete plastid genome

“…Its habitat and morphology have obviously undergone high specializations and extensive reductions ( Gustafsson et al 2002 ). These species lack double fertilization and the vascular tissue is reduced or lost ( Johri et al 1992 ; Kato 2016 ; Khanduri et al 2016 ). The vegetative body of most river-weeds resembles algae, lichens, or mosses.…”

“…The life cycle of river-weeds is dictated by high and low water periods ( Lalruatsanga 2020 ). During the rainy season, the plants submerge in violent currents exclusively in the vegetative phase ( Kato 2016 ). When the water level lowers during the dry season, the plants emerge from the water and enter into the reproductive phase ( Kelly et al 2010 ; Koi and Kato 2012 , 2015 ).…”

“…They are annual herbs, but perennial when submerged all year round ( Kato et al 2019 ). Exposure is necessary for reproduction, otherwise the plants will be unable to complete their life cycle ( Kato 2016 ).…”

Zeylanidium manasiae, a new species of Podostemaceae based on molecular and morphological data from Kerala, India

“…Podostemaceae is subdivided into three monophyletic subfamilies: Podostemoideae, Tristichoideae and Weddellinoideae (monogeneric and monospecific) (Koi et al 2012). Southern Asia is one of the main centres of diversity for the podostemads, accounting for 17 genera and 80 species from the region (Kato 2016). India harbours 28 species of Podostemaceae, in which 23 are endemics (Khanduri et al 2014).…”

A synoptic revision of Inversodicraea (Podostemaceae)

“…There are about 300 species globally, in c. 54 genera (Koi et al 2012, Kato 2016. Species numbers are highest in tropical America, followed by Asia, with Africa having c. 80 species.…”