dictyNews
Electronic Edition
Volume 44, number 6
February 23, 2018
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Abstracts
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Protists and Multiple Routes to the Evolution of Multicellularity
Vidyanand Nanjundiah1, Iñaki Ruiz-Trillo 2, and David Kirk3
1. Centre for Human Genetics, Bangalore, India; [log in to unmask]
2. Institut de Biologia Evolutiva, Barcelona, Spain; [log in to unmask]
3. Washington University, St. Louis, USA; [log in to unmask]
Chapter 4, History of Evolutionary Cell Biology (eds)
B Hall and S Moody, eds., CRC Press, in press
Complex multicellular eukaryotic forms evolved independently at different
moments in life’s history, around 200- 800 Myr ago. We ask what aspects
pertaining to the evolutionary origin of multicellular life can be inferred from
a survey of otherwise dissimilar protists that display one or both of two
features: a unicellular-to-multicellular transition as part of their normal life
cycle, or membership of a closely related group that contains both unicellular
and multicellular members. A likely answer comes from an examination of
representative species from three major groups of life, the Amoebozoa
(cellular slime moulds), Opisthokonta (unicellular holozoans and metazoans)
and Archaeplastida (volvocine green algae). The following inferences can be
drawn from features that are common to the three cases. (i) Naïve ideas of
what is simple (=“primitive”) and what is complex (=“evolved”), primarily
based on morphology, bear no relation to what can be inferred as ancestral
and derived states on the basis of DNA-based phylogeny. In short, grades of
organizational complexity need not necessarily reflect clades of closest
relatives. (ii) Cells with the same genome or similar genomes can become
multicellular in more than one way, or go through the multicellular phase in
different ways, or display a variety of multicellular forms. In the cellular slime
moulds, which display aggregative multicellularity, species belonging to one
clade can mimic what were believed on morphological grounds to be diverse
genera. Among the unicellular holozoans, choanoflagellates form clonal
colonies, filastereans aggregate and teretosporeans form a coenocyte.
In contrast, there is no evidence that aggregation has ever led to true
multicellularity in the volvocine algae. (iii) Volvocine algae provide a dramatic
example of temporal differentiation giving way to spatial differentiation once a
critical size (= number of cells) is exceeded. The cellular slime molds too
show size-dependent morphologies and developmental patterns, though not
as strikingly. (iv) Given that single-celled ancestors seem to have possessed
many of the protein-coding genes that were believed to be specific to
metazoans, all that may have been required for a unicellular form to ‘go
multicellular’ may have been an environmental trigger (e. g., an increase in
atmospheric oxygen content) that permitted size increase that, among other
things, was a defense against predation. (v) Alternatively, environmental
changes may have fostered the origin of multicellular forms from pre-existing
cellular interaction systems; genetic changes may have arisen secondarily
by way of ensuring developmental reliability. (vi) In those cases in which
embryonic development arose (i.e., embryophites and metazoans), there
could be a combination of all those causes, as well as the evolution of new
major genomic regulatory capabilities, such as distal regulation.
submitted by: Vidya Nanjundiah [ [log in to unmask]]
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[End dictyNews, volume 44, number 6]
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