Aberrant clones: Birth order generates life history diversity in Greater Duckweed, Spirodela polyrhiza

Abstract

Environmental unpredictability is known to result in the evolution of bet‐hedging traits. Variable dormancy enhances survival through harsh conditions, and is widely cited as a diversification bet‐hedging trait. The floating aquatic plant, Spirodela polyrhiza (Greater Duckweed), provides an opportunity to study diversification because although partially reliable seasonal cues exist, its growing season is subject to an unpredictable and literally “hard” termination when the surface water freezes, and overwinter survival depends on a switch from production of normal daughter fronds to production of dense, sinking “turions” prior to freeze‐over. The problem for S. polyrhiza is that diversified dormancy behavior must be generated among clonally produced, genetically identical offspring. Variation in phenology has been observed in the field, but its sources are unknown. Here, we investigate sources of phenological variation in turion production, and test the hypothesis that diversification in turion phenology is generated within genetic lineages through effects of parental birth order. As expected, phenotypic plasticity to temperature is expressed along a thermal gradient; more interestingly, parental birth order was found to have a significant and strong effect on turion phenology: Turions are produced earlier by late birth‐order parents. These results hold regardless of whether turion phenology is measured as first turion birth order, time to first turion, or turion frequency. This study addresses a question of current interest on potential mechanisms generating diversification, and suggests that consistent phenotypic differences across birth orders generate life history variation.

Skimming the surface: duckweed as a model system in ecology and evolution

“…Furthermore, the short lifespan of individual ramets makes it possible to track cohorts longitudinally and even multigenerationally, allowing in a matter of months the collection of data that would take years to obtain even in annual plants. For example, the relative ease of tracking cohorts over multiple generations has made duckweeds ideal for studying parental age effects in the contexts of both senescence (Ashby and Wangermann, ; Barks and Laird, ) and bet hedging (Mejbel and Simons, ).…”

Birth order as a source of diversification in spring phenology and its potential effects on performance in greater duckweed, Spirodela polyrhiza

“…These studies argue that the expression of these adaptive traits allows L. inflata to persist through variable and unpredictable environments with little to no genetic variation among offspring. Similarly, putative diversification bet hedging has been observed in clone populations of the floating aquatic plant Spirodela polyrhiza (L.) Schleiden, which despite genetic homogeneity, produces offspring exhibiting diverse phenotypes (Compton 2013;Mejbel and Simons 2018). This quality of Spirodela polyrhiza will be the focus of this research.…”

“…Changes in temporally variable environments include (but are not limited to) biotic fluctuations, as well as abiotic fluctuations in temperature, precipitation, and photoperiod (Childs et al 2010;Brewer et al 2017). These abiotic environmental changes are comprised of deterministic and stochastic components (Graham et al 2014;Botero et al 2015;Mejbel and Simons 2018) and can lead to climate differences across the globe; for example, semi-regular wet-dry cycles near the equator, cold winters and warm summers in the mid-latitudes, and mild bright summers and cold dark winters near the poles.…”

“…The efficacy of adaptive tracking is thus limited not only by the availability of genetic variation, but by the severity and rate of environmental change. The literature notes two other modes of adaptation to variable environments: phenotypic plasticity (Via and Lande 1985;Bell and Collins 2008;Simons 2011;Botero et al 2015;Mejbel and Simons 2018) and bet hedging (Seger and Brockmann 1987;Philippi and Seger 1989;Bell and Collins 2008;Simons 2011;Starrfelt and Kokko 2012;Botero et al 2015;Mejbel and Simons 2018). It should be noted that these modes of response also evolve through adaptive tracking but over longer time scales (Simons 2011) even if they appear to "prepare" for future environments.…”

“…Furthermore, duckweed species are among the fastest growing angiosperms in the world in terms of biomass production (Kutschera and Niklas 2015). They have been the focus of studies in physiology (Jacobs 1947;Perry 1968;Appenroth et al 1989Appenroth et al , 1996Ley et al 1997;Lemon et al 2001), toxicology (Appenroth et al 2000;Oláh et al 2015Oláh et al , 2018Geng et al 2018;Singh et al 2018), and evolutionary biology (Vasseur and Aarssen 1992; Laird 2015, 2016;Ankutowicz and Laird 2018;Mejbel and Simons 2018) as well as in industrial contexts with applications in wastewater treatment Ziegler et al 2017;Kaur et al 2018;Singh et al 2018), pharmaceuticals , human consumption Appenroth et al 2017;Kaur et al 2018), and biofuel crop production Gönen 2018;Kaur et al 2018). Duckweed, although sometimes still considered to form the family Lemnaceae,…”