Multicellularity As An Evolutionary Advance

Published Date: 02 Nov 2017

The transition towards multicellularity from unicellular organisms represents a major transition in evolution of complex organisms. (Pfeiffer and Bonhoeffer, 2003; Bonner, 1998) The evolution of multicellularity has lead to many descendants including plants, fungi and animals. (Willensdorfer, 2009) Multicellularity results in two main benefits, size related benefits as well as functional specialization and division of labour, which increase the survival fitness of organisms. Therefore multicellularity should be regarded as an evolutionary advance. (Grosberg and Strathmann, 2007)

Multicellularity has arisen independently several times during the course of evolution. This suggests that multiple strong driving factors favour the transition to multicellularity and that the genetic and developmental obstacles opposing this transition are relatively easily overcome. (Pfeiffer and Bonhoeffer, 2003; Grosberg and Strathmann, 2007)

Many studies indicate that the advantages of increased size associated with multicellularity favour the origination and persistence of multicellularity. (Grosberg and Strathmann, 2007) The first and simplest multicellular organisms were likely clusters of simple, undifferentiated cells. (Pfeiffer and Bonhoeffer, 2003; Grosberg and Strathmann, 2007; Willensdorfer, 2009). These clusters of simple, undifferentiated multicellular organisms can arise by two means: cell division followed by incomplete separation which is common in multicellular organisms with aquatic origin e.g. volvocine algae; or by aggregation of solitary cells which is common in multicellular organisms with terrestrial origin e.g. in social amoebae (Bonner, 2003 ; Wolpert and Szathmáry, 2002)

A study entitled "The Origins of Multicellularity" conducted by Bonner proposed that the first step in the evolution of multicellularity was a size increase which arose accidently by a chance mutation. If the larger cell mass held any advantages, the mutation would then be retained by the process of natural selection and lead in the evolution of multicellularity. (Bonner, 1998)

Increased size may be beneficial for survival as it results in: reduced predation; the ability to catch larger prey; an internal environment protected by a layer of external cells; more efficient reproduction (higher number and quality of offspring); provides storage reserves; expands feeding opportunities; allows for novel metabolic opportunities and enhanced mobility.(Michod et al , 2006; Grosberg and Strathmann, 2007)

For example Scenedesmus acutus, a green algae, reduces its risk of predation by inducing multicellular forms in environments in which its predators exist. S. acutus can exist either as unicells or colonies and is found to exist mainly in colonial form in nature, but unicellular forms in lab cultures. If unicellular S. acutus cultures are exposed to water from cultures of Daphnia (a predator of S. acutus) colony formation increases significantly. (Grosberg and Strathmann, 2007)

A study entitled "An evolutionary scenario for the transition to undifferentiated multicellularity" conducted by Pfeiffer and Bonhoeffer used computer simulations to confirm that cooperation in the use of external resources lead to direct benefits for cooperators and therefore may bring about the evolution of simple undifferentiated cell clusters. Cooperative behaviour is an evolutionary advantage as among cooperating individuals where efficient resource use takes place, benefits are high for all individuals. While non-cooperative behaviour of fast but inefficient resource use results in benefits confined to the individual and the disadvantages are shared by all. Clusters of simple, undifferentiated cells may evolve in a process by which cooperating individuals reduce their interactions with non-cooperating individuals resulting in increased individuality of the cell group. (Pfeiffer and Bonhoeffer, 2003)

An important evolutionary advantage of multicellularity is that it allows for functional specialization and division of labour. Unicellular organisms have the potential to differentiate leading to increased variety in their population but they are limited as they can only divide their labour in terms of time, whereas multicellular organisms can divide tasks among differentiated cells.(Grosberg and Strathmann, 2007)

Division of labour is critical as some metabolic processes are incompatible. (Grosberg and Strathmann, 2007) Cyanobacteria are organisms which undergo both photosynthesis and nitrogen fixation. This is problematic because nitrogen fixation can only occur in the absences of oxygen, but oxygen is a by product of photosynthesis, therefore the two processes cannot take place simultaneously within the same cell. Some cyanobacteria solve this problem by photosynthesising during the day and fixing nitrogen at night. However, another more innovative approach involves some cells in the filament specializing for nitrogen fixation. These nitrogen fixing specialized cells called heterocysts lack chlorophyll and have thicker, less permeable cell walls than photosynthetic cell to prevent oxygen entry. The nitrogen products produced in heterocysts are then exchanged for nutrients from photosynthetic cells via pores in heterocysts walls. Furthermore heterocysts are economically spaced for optimal efficiency. (Bonner, 1998) This shows cell-cell interactions and developmental coordination important in multicellular organisms. (Grosberg and Strathmann, 2007)

Volvocine algae are another example of where multicellularity is beneficial by allowing for functional specialization and division of labour. (Grosberg and Strathmann, 2007) The evolutionary fitness of an organism can be interpreted in terms of survival and reproduction.(Michod et al , 2006) In unicellular organisms, the single cell must account for both fitness components. Whereas multicellular organisms can have cells specialized in both components, therefore have differentiation and specialization of reproductive(germ) and survival enhancing (soma) functions.(Michod et al , 2006)Volvocine algae exist in a range of forms including unicellular forms, multicellular forms with no or incomplete germ-soma differentiation and multicellular forms with complete germ-soma differentiation.(Michod et al , 2006) Differentiated multicellular forms such as Volvox consist of two different cell types: biflagellate somatic cells which allow the colony to move and reproductive cells, or germ cells, which divide to produce daughter colonies. Somatic cells are terminally differentiated and incapable of dividing once formed, while germ cells are specialized for cell division and are incapable of mobility. (Bonner, 2003) This pattern of germ-soma separation and differentiation has evolved repeatedly in volvocine algae suggesting that strong selective forces drive the evolution of multicellularity and germ-soma specialization.(Michod et al , 2006 ; Grosberg and Strathmann, 2007) It should be noted that only a few genetic changes are involved in this transition.(Bonner, 2003) A study entitled "Life-history evolution and the origin of multicellularity" conducted by Michod et al concluded that trade-offs between survival and reproduction during the evolution of multicellularity select for the specialization of germ and soma; and that advantages of division of labour increase, as colony size increases. (Michod et al , 2006)

All organisms possess mutations which cause variations in inherited traits. Evolution results from the competition between slightly different variations of organisms and consequentially the selection and proliferation of variants with increased survival fitness. Multicellularity clearly increases the survival fitness of organisms and therefore has an evolutionary advantage; opening the door to numerous possibilities, not possible in unicellular organisms. (Willensdorfer, 2009) It is therefore concluded that multicellularity should be regarded as an evolutionary advance.