Dissertation Proposal - The University of Arizona Campus Repository

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Volvocalean green algae are negatively buoyant. They need motility to avoid sinking and to reach light and nutrients (Hoops 1997; Kirk 1998; Koufopanou 1994). The dramatic change in the flagellar apparatus between unicellular species and species that form colonies is still further evidence of how important motility is for the Volvocales (Hoops 1997). Thus, the constraints and opportunities of flagellar motility may have been the major driving force as colonies increased in size during the evolutionary transitions from multicellular colonies with no cellular differentiation to multicellular colonies with germ-soma separation. Due to the Volvocales’ peculiar mode of development (explained below, Section II), as colonies increase in size their motility capabilities may be affected as a result of two different biological factors: in undifferentiated colonies motility is negatively affected during cell division (the flagellation constraint; Koufopanou 1994), and larger colonies need to invest more in somatic cells for self-propulsion and avoid sinking (the flagellar motility hypothesis, presented in this dissertation). In algae and other microorganisms motility can be associated not only with translocation, but also with the enhancement of molecular transport, improving the acquisition of molecules important for maintaining productivity, and increasing the dispersal of waste products beyond the range of inadvertent diffusive recycling (Niklas 1994; 2000). Since larger colonies have higher nutrient requirements, the collective flagellar beating of somatic cells also serves for boundary layer stirring and remote transport of nutrients. This is fundamental to maintain a high nutrient concentration gradient between the medium and the colony. 12
To summarize, as volvocalean colonies increase in size, the increase in cell specialization observed in extant species can be explained in terms of the need for increased motility capabilities. To investigate this hypothesis I develop a model based on standard hydrodynamics that describes the physical factors involved in motility in these organisms. I then measure under controlled laboratory conditions the motility (self- propulsion) of the different colony types as well as the parameters and other variables used in the model. I am the first to provide comparative measurements of swimming speeds of an algal lineage composed of organisms of different sizes and degrees of complexity. To test the importance of collective flagellar beating on nutrient uptake, I design an experiment that quantifies the effect of advective dynamics on the productivity of the colonies. In short, I show that larger colonies need to invest in somatic cells that specialize in motility in order to remain motile and avoid sinking while reproducing (Section III). This need arises from the enlargement of reproductive cells to form daughter colonies, which increases the mean mass of the colony. As colonies increase in size, a higher proportion of somatic cells is required (i.e., increased somatic to reproductive cell ratio). Once larger colonies have a high proportion of somatic cells they are better off with a non-flagellated germ cell that specializes in reproduction. The specialized germ also serves to increase motility, since packaging it inside the colony minimizes drag. More investment in somatic cells for motility has the additional and important benefit of enhanced molecular transport of nutrients and wastes thanks to the flow created by collective flagellar beating (Section IV). 13
Page 1 and 2: A HYDRODYNAMICS APPROACH TO THE EVO
Page 3 and 4: STATEMENT BY AUTHOR This dissertati
Page 5 and 6: TABLE OF CONTENTS - Continued IV -
Page 7 and 8: LIST OF TABLES Table 1, Development
Page 9 and 10: eproductive cell ratio must increas
Page 11: and fecundity (e.g. higher number o
Page 15 and 16: differentiation, e.g., Gonium and E
Page 17 and 18: for up to 5 cell divisions (e.g., t
Page 19 and 20: III - CELLULAR DIFFERENTIATION AND
Page 21 and 22: In GS and GS/S colonies, q = 1 sinc
Page 23 and 24: N ≈ 4 π r 3 3 max 4 3 3 π[(1
Page 25 and 26: in higher proportions of somatic ce
Page 27 and 28: To investigate the assumptions used
Page 29 and 30: EXPERIMENTAL METHODS Species and Mu
Page 31 and 32: Colonies were sampled from the sync
Page 33 and 34: Data Analysis To perform a size-dep
Page 35 and 36: Figure 4. The change in size and sw
Page 37 and 38: Figure 5. Allometric analysis: R,
Page 39 and 40: Log R, Vsed, and Vup versus Log N u
Page 41 and 42: colonies develop, larger cells have
Page 43 and 44: I now compute a physical limit on c
Page 45 and 46: Table 5. Models selected from the M
Page 47 and 48: IV - CELLULAR DIFFERENTIATION AND F
Page 49 and 50: y the flagellar beating of somatic
Page 51 and 52: light period and germ cells continu
Page 53 and 54: medium treatments (total of four).
Page 55 and 56: the still and bubbling medium exper
Page 57 and 58: entrained fluorescent flow-marking
Page 59 and 60: arrangement of flagellar motors on
Page 61 and 62: confirming the hypothesis that coll
Page 63 and 64:
concurrently benefits nutrient upta
Page 65 and 66:
and in colony radius) decrease the
Page 67 and 68:
APPENDIX A: VELOCITIES AND SIZES. T
Page 69 and 70:
Vc 600 5 22 193 15.6 12 114 20.0 12
Page 71 and 72:
APPENDIX B: COMPLEMENTARY ANALYSIS
Page 73 and 74:
Induced Hatching Experiments In a s
Page 75 and 76:
Size and Flagellar Length I measure
Page 77 and 78:
APPENDIX C: ANALYSIS OF R AND ∆M
Page 79 and 80:
REFERENCES Angeler, D. G., M. Schag
Page 81:
(Volvocales). Biologia 58:425-431.
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