Asymmetry and integration of cellular morphology in Micrasterias compereana

Jiří Neustupa

doi:10.1186/s12862-016-0855-1

Abstract

BackgroundUnicellular green algae of the genus Micrasterias (Desmidiales) have complex cells with multiple lobes and indentations, and therefore, they are considered model organisms for research on plant cell morphogenesis and variation. Micrasterias cells have a typical biradial symmetric arrangement and multiple terminal lobules. They are composed of two semicells that can be further differentiated into three structural components: the polar lobe and two lateral lobes. Experimental studies suggested that these cellular parts have specific evolutionary patterns and develop independently. In this study, different geometric morphometric methods were used to address whether the semicells of Micrasterias compereana are truly not integrated with regard to the covariation of their shape data. In addition, morphological integration within the semicells was studied to ascertain whether individual lobes constitute distinct units that may be considered as separate modules. In parallel, I sought to determine whether the main components of morphological asymmetry could highlight underlying cytomorphogenetic processes that could indicate preferred directions of variation, canalizing evolutionary changes in cellular morphology.ResultsDifferentiation between opposite semicells constituted the most prominent subset of cellular asymmetry. The second important asymmetric pattern, recovered by the Procrustes ANOVA models, described differentiation between the adjacent lobules within the quadrants. Other asymmetric components proved to be relatively unimportant. Opposite semicells were shown to be completely independent of each other on the basis of the partial least squares analysis analyses. In addition, polar lobes were weakly integrated with adjacent lateral lobes. Conversely, higher covariance levels between the two lateral lobes of the same semicell indicated mutual interconnection and significant integration between these parts.ConclusionsMicrasterias cells are composed of several successively disintegrated parts. These integration patterns concurred with presumed scenarios of morphological evolution within the lineage. In addition, asymmetric differentiation in the shape of the lobules involves two major patterns: asymmetry across the isthmus axis and among the adjacent lobules. Notably, asymmetry among the adjacent lobules may be related to evolutionary differentiation among species, but it may also point out developmental instability related to environmental factors.

Highlights

Unicellular green algae of the genus Micrasterias (Desmidiales) have complex cells with multiple lobes and indentations, and they are considered model organisms for research on plant cell morphogenesis and variation
The analyses demonstrated that there are two dominant types of morphological asymmetry among the terminal lobules of the cells
In summary, this study illustrates that Micrasterias cells are composed of parts with widely different levels of morphological integration

Summary

Introduction

Unicellular green algae of the genus Micrasterias (Desmidiales) have complex cells with multiple lobes and indentations, and they are considered model organisms for research on plant cell morphogenesis and variation. Micrasterias cells have a typical biradial symmetric arrangement and multiple terminal lobules They are composed of two semicells that can be further differentiated into three structural components: the polar lobe and two lateral lobes. In organisms with modular body plans, composed of multiple repeated parts, the concepts of morphological symmetry and integration are inherently related Typical organisms with this architecture are vascular plants with multiple repeated organs symmetric to each other [1, 2]. Parts that develop diachronically, e.g. in different life-cycle phases or ecological conditions, may profoundly differ in their morphological integration levels Such patterns may constitute key constraints in the evolution of biological forms, because they facilitate evolutionary change in only a part of a structure, leading to asymmetric differentiation of shape features. Evolvability of organism morphology is constrained both by translational symmetry, i.e. multiple repetitions of symmetric parts based on joint developmental networks at the molecular level, and by different integration levels among the different regions

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