$68,040 – $92,343 pa (plus 9.5% employer superannuation)
Full-time, starting early 2020
One year, fixed term with the possibility of extension to a second year
Monash University Clayton campus
Professor Dustin Marshall is seeking an experienced ecologist / evolutionary biologist, who specialises in microalgal biology with a strong empirical background, to explore the ways in which size affects the structure and function of marine phytoplankton. This position will be with the Centre for Geometric Biology within the School of Biological Sciences at Monash University.
As the successful candidate, you will be expected to maintain the Centre’s evolved lines of the microalgae Dunaliella and use these evolved microalgae to undertake experiments that test ecological and evolutionary theories. You will also have a strong quantitative background and have a demonstrated track record in producing high-quality publications.
Key selection criteria
A doctoral qualification in empirical ecology / evolutionary biology using microalgae as a model species.
Demonstrated analytical and manuscript preparation skills; including an excellent track record of refereed research publications in high impact journals.
Demonstrated experience in empirical research using cutting-edge quantitative approaches.
Strong leadership, organisational and project management skills.
$66,706 to $90,532 pa + 9.5% employer superannuation
Full-time, starting late 2018
Monash University Clayton campus
Professor Dustin Marshall is seeking a marine larval biologist, with strong quantitative skills, to explore the ways in which temperature affects the energetics of development in marine invertebrates. This position will be with the Centre for Geometric Biology within the School of Biological Sciences at Monash University.
As the successful candidate, you will be expected to undertake experiments to determine the relative performance of different larval types across every stage of the life history, but more importantly demonstrate a strong conceptual understanding of relevant life history theory and have a demonstrated track record in producing high quality publications.
Key selection criteria
A doctoral qualification in larval biology
Demonstrated analytical and manuscript preparation skills; including an excellent track record of refereed research publications in high impact journals
Demonstrated experience in empirical research using cutting-edge quantitative approaches
Ability to solve complex problems by using discretion, innovation and the exercise of diagnostic skills and/or expertise
Well-developed planning and organisational skills, with the ability to prioritise multiple tasks and set and meet deadlines
Excellent written communication and verbal communication skills with proven ability to produce clear, succinct reports and documents
A demonstrated awareness of the principles of confidentiality, privacy and information handling
A demonstrated capacity to work in a collegiate manner with other staff in the workplace
Demonstrated computer literacy and proficiency in the production of high level work using software such as Microsoft Office applications and specified University software programs, with the capability and willingness to learn new packages as appropriate.
Enquiries to Professor Dustin Marshall on +61 3 9902 4449
Classic theories such as the ‘selfish gene hypothesis’ are not always easy to test. If true, then the theory proposes that cells accumulate extra DNA over time even when they provide no benefit to the organism – selfish genes. This means individuals with more DNA relative to their cell size should have reduced fitness. What is more, because the accumulation of DNA creates a burden on the organism, a good evolutionary strategy should be to reduce the amount of redundant DNA over time.
In one of the first tests of the theory using a single species, Martino Malerba, Giulia Ghedini and Dustin Marshall have found direct evidence that reduced amounts of total DNA (genome size) is associated with fitness benefits in a species of marine microalgae.
Scientists face a problem when they compare cell size and genome size between different species. It is hard to separate whether the slower rates of metabolism, development and growth are a result of genome size or the fact that larger organisms tend to have a slower pace of life anyway.
Martino and his colleagues were able to overcome this issue by using their evolved lines of the marine phytoplankton Dunaliella tertiolecta where cells have been artificially selected to be large or small. They were able to use a staining technique to measure the amount of DNA content within cells of different sizes but, importantly, of the same species and the same evolutionary age.
They weren’t surprised to find larger cells had more DNA. But they also found that, in cells of the same size, the cells with smaller genomes grew faster and accumulated more biomass; that is, had greater fitness.
So, this first finding was consistent with the prediction that reducing the amount of DNA in a cell can have positive effects for the fitness of a species. But having demonstrated that minimizing DNA can improve fitness, the team then wanted to test the prediction that species should decrease their DNA content as they evolve. Again, they were able to test this by using the Dunaliella and monitoring the evolving lines for a year or approximately100 generations.
Their results confirmed that cells decreased their DNA content by up to 11% across 100 generations of evolution. However, they also found that cells with already low amounts of DNA showed no change over time, which suggests the existence of an absolute lower limit in the DNA content of this species.
Overall, they have direct evidence for fitness benefits associated with reduced relative genome size consistent with the selfish gene hypothesis and as well as a minimum genome size below which an organism can’t maintain functionality.
Authors: Martino E Malerba, Giulia Ghedini, and Dustin J Marshall
Published in:Current Biology
Genome size is tightly coupled to morphology, ecology, and evolution among species, with one of the best-known patterns being the relationship between cell size and genome size.
Classic theories, such as the ‘selfish DNA hypothesis,’ posit that accumulating redundant DNA has fitness costs but that larger cells can tolerate larger genomes, leading to a positive relationship between cell size and genome size. Yet the evidence for fitness costs associated with relatively larger genomes remains circumstantial.
Here, we estimated the relationships between genome size, cell size, energy fluxes, and fitness across 72 independent lineages in a eukaryotic phytoplankton. Lineages with relatively smaller genomes had higher fitness, in terms of both maximum growth rate and total biovolume reached at carrying capacity, but paradoxically, they also had lower energy fluxes than lineages with relative larger genomes. We then explored the evolutionary trajectories of absolute genome size over 100 generations and across a 10-fold change in cell size.
Despite consistent directional selection across all lineages, genome size decreased by 11% in lineages with absolutely larger genomes but showed little evolution in lineages with absolutely smaller genomes, implying a lower absolute limit in genome size.
Our results suggest that the positive relationship between cell size and genome size in nature may be the product of conflicting evolutionary pressures, on the one hand, to minimize redundant DNA and maximize performance — as theory predicts — but also to maintain a minimum level of essential function.
Malerba ME, Ghedini G, Marshall DJ (2020) Genome size affects fitness in the eukaryotic alga Dunaliella tertiolecta. Current BiologyPDFDOI
While travel restrictions have become part of the new normal for people all around the world, a recent study has found that the distance travelled by marine larvae is dictated by both biological and physical constraints.
Marine invertebrates face many challenges when it comes to reproduction. Sperm and sometimes eggs are released into the water where they must meet-up to allow fertilisation to take place. These fertilised embryos develop into larvae and remain in the water column until they find a suitable spot to settle. The amount of time they spend in the water column and the distances they travel can be vastly different for different species.
It is not easy to measure how far larvae travel in real-time so, instead, biologists often use genetic information to work out the relatedness of populations as a proxy for dispersal distance. An alternative approach gathers data on larval characteristics to estimate the time spent in the plankton and so the potential for dispersal.
Mariana Noriega and Dustin Marshall from the Centre for Geometric Biology have been working with colleagues from the United States to examine existing data to help them grasp how larval dispersal distance changes on a global scale. Recent exploration of this question has focused on the role of latitude (or temperature) on larval development, developmental mode (feeding or non-feeding larvae), maternal investment into egg size and hydrodynamics. Often these factors are considered separately rather than all together.
Here’s what we know. Higher temperatures speed up larval development so larvae in the tropics may spend less time in the plankton and disperse less far. But to complicate things, larvae in the tropics are more likely to be feeding larvae which means they tend to spend more time in the plankton than their non-feeding counterparts. Plus, mothers in cooler climes tend to invest more energy into their eggs which for non-feeding larvae means more time in the plankton for those that live at higher latitudes.
Mariana and her colleagues were particularly interested in understanding whether these life-history traits that change with latitude will combine with ocean current information to support their prediction that dispersal distances are shorter in the tropics.
The team have looked at data from 766 marine invertebrate species and classified the larvae into feeding or non-feeding. They extracted data on egg size and the time spent in the plankton, plus the latitude and longitude of the recorded observation.
They were then able to use statistical models to estimate planktonic duration at different latitudes by incorporating their data on development mode and egg size. Having the location of the record also enabled Mariana and the team to estimate local current speeds using the publicly available Mercator-Ocean modelling system. Finally, the expected planktonic duration for the ‘average larvae’ was then multiplied by current speed at each location to estimate dispersal potential.
To the team’s surprise, they didn’t find that dispersal distances were shorter in the tropics.
Instead, they found that the faster surface current speeds in the tropics overcame the effects of temperature on larval development time. So, even though larvae spend less time in the plankton they still have the potential to disperse further than the team predicted due to the faster current speeds.
In fact, the team found that larvae travel further at high and low latitudes, that is, the tropics and the poles. Dispersal distances were shortest in temperate regions where the time spent in the plankton is intermediate and current speeds are slower.
Species richness is greater in the tropics but it seems as if this pattern is not driven by larval dispersal as has been previously suggested. If species richness were driven purely by dispersal distance, this study suggests we would find similar species richness at high latitudes and in the tropics, yet this is not the case.
Understanding patterns in larval dispersal is essential for understanding patterns in marine biodiversity and managing our marine systems. Without this, we will struggle to adequately design marine protected areas, effectively manage biological invasions and predict the consequences of climate change.
Authors: Mariana Álvarez-Noriega, Scott C Burgess, James E Byers, James M Pringle, John P Wares, and Dustin J Marshall
Published in:Nature Ecology & Evolution
The distance travelled by marine larvae varies by seven orders of magnitude. Dispersal shapes marine biodiversity, and must be understood if marine systems are to be well managed.
Because warmer temperatures quicken larval development, larval durations might be systematically shorter in the tropics relative to those at high latitudes. Nevertheless, life history and hydro-dynamics also covary with latitude—these also affect dispersal, precluding any clear expectation of how dispersal changes at a global scale.
Here we combine data from the literature encompassing >750 marine organisms from seven phyla with oceanographic data on current speeds, to quantify the overall latitudinal gradient in larval dispersal distance.
We find that planktonic duration increased with latitude, confirming predictions that temperature effects outweigh all others across global scales. However, while tropical species have the shortest planktonic durations, realized dispersal distances were predicted to be greatest in the tropics and at high latitudes, and lowest at mid-latitudes. At high latitudes, greater dispersal distances were driven by moderate current speed and longer planktonic durations. In the tropics, fast currents overwhelmed the effect of short planktonic durations.
Our results contradict previous hypotheses based on biology or physics alone; rather, biology and physics together shape marine dispersal patterns.
Álvarez-Noriega M, Burgess SC, Byers JE, Pringle JM, Wares JP, Marshall DJ (2020) Global biogeography of marine dispersal potential. Nature Ecology & Evolution PDFDOI
It is generally recognised that animals perform best at certain temperatures, so-called optimal temperatures. To understand how measures of performance, such as growth or running ability, will change with changing temperatures we need to understand the physiological processes limiting performance.
One compelling theory, known as the oxygen and capacity-limited thermal tolerance (OCLTT), suggests that a reduction in oxygen availability limits performance.
As temperature shifts away from optimal, the demand for oxygen in tissues is thought to outpace the rate of supply. This means a shift to anaerobic metabolism, a process far less efficient than aerobic metabolism, causing a reduction in performance.
Emily Lombardi, Candice Bywater and Craig White wanted to test the theory and used the speckled cockroach as their experimental animal because they are easy to breed and keep in the lab. Unlike other studies testing the OCLTT hypothesis, their interest was in the less extreme ends of the temperature range, which the OCLTT hypothesis specifically addresses.
First, Emily and her colleagues calculated the temperature that optimised growth by allowing juvenile cockroaches to develop for 35 days at 8 temperatures ranging between 10 and 36 °C. They also determined the temperatures (both above and below the optimal temperature) where growth was reduced by 32%. These 3 temperatures (optimal, lower and upper developmental temperatures) were then used in oxygen manipulation experiments.
Next, they ran an experiment including the three temperatures plus three oxygen concentrations; atmospheric (21%), hypoxic (10%) and hyperoxic (40%). Cockroaches were assigned to one of the 9 treatments and growth rate was measured repeatedly over 5 weeks. Running performance – time on a treadmill – was measured at the end of the 5 weeks at the three temperatures. Tracheal morphology was also quantified because, in some species, changes to the oxygen delivery system can alleviate the demand for increased oxygen supply.
If the OCLTT theory held, then the team expected to see a difference in performance at the different oxygen concentrations at each temperature. But, instead, they found increasing oxygen concentrations did not mitigate the effects of sub-optimal temperatures. There was also no evidence that tracheal morphology changed as a result of the developmental temperature or oxygen environment. The team concluded that oxygen supply was not the main determinant of temperature-related performance limitations for the speckled cockroach.
It seems, the OCLTT may not provide the unifying theory its proponents hoped, but instead, a species’ thermal tolerance is likely shaped by a range of factors.
We know that the rate at which organisms use energy (metabolic rate) varies substantially between individuals of the same species, even after accounting for size and temperature. What we are less sure about, is, why we see this variation.
When Amanda Pettersen and her colleagues thought about this question they considered it plausible that the competitive environments that individuals find themselves in might be important in determining whether a faster or slower metabolic rate is selected for. They wanted to find out whether variation in competition affects selection on metabolic rates and whether that could account for the variation in metabolic rate that persists more generally.
To test this idea Amanda used the model species Bugula neritina because it allowed the team to collect larvae in the lab, measure size and metabolic rate of larvae before assigning the larvae to one of three ‘competition’ treatments. Once the larval measurements were made, larvae were randomly assigned to either a no-competition environment, a competitive environment where they were put with other Bugula neritina (intraspecific competition) or, a competitive environment with an established mixed-species community (interspecific competition).
Individuals were then returned to the field and monitored weekly for survival, growth, age when first become reproductive and fecundity (total reproductive output).
The team already knew that higher metabolic rates are linked to faster growth, earlier onset of reproduction, and a shorter lifespan, while low metabolic rates are associated with a slow pace-of-life (slow growth, late onset of reproduction and long lifespan).
This experiment showed that individuals with higher metabolic rates were more likely to survive, more likely to reproduce and had greater numbers of ovicells at the start of reproduction, in the more intense, interspecific competition treatment. The team speculates that the individuals with higher metabolic rates grow more quickly enabling them to reach resources such as food and oxygen that are less available to smaller, slower growing organisms. There is a downside: these higher metabolic rate individuals also had a shorter lifespan.
The expectation amongst evolutionary biologists is that where there is strong selection pressure for a particular trait then the variation within that trait is reduced. So even accounting for the shorter lifespan, the overall increase in reproduction should mean that a higher metabolic rate is strongly selected for in competitive environments.
But the team also found that individuals with lower metabolic rates had a higher probability of living for longer in the absence of competition and would have continued to reproduce long after the individuals with a higher metabolic rate had died.
Bugula live on hard substrates and these areas commonly vary in their composition. Complex three-dimensional, highly diverse communities are interspersed with sparse populations of a few species and patches of bare areas. Individuals with lower metabolic rates that happen to arrive at a bare patch will have a long reproductive period and high overall fecundity. In contrast, individuals with a high metabolic rate that settle in amongst other species will be reproductive quickly and so successfully produce large numbers of offspring.
Because Bugula larvae are likely to find themselves in a variety of different competitive environments – even when they settle relatively close to each other – there is a strong evolutionary argument to explain the persistence of variation in metabolic rate, for Bugula at least.
Authors: Amanda K Pettersen, Matthew D Hall, Craig R White, and Dustin J Marshall
Published in:Evolution Letters
Metabolism is linked with the pace-of-life, co-varying with survival, growth, and reproduction. Metabolic rates should therefore be under strong selection and, if heritable, become less variable over time. Yet intraspecific variation in metabolic rates is ubiquitous, even after accounting for body mass and temperature.
Theory predicts variable selection maintains trait variation, but field estimates of how selection on metabolism varies are rare.
We use a model marine invertebrate to estimate selection on metabolic rates in the wild under different competitive environments.
Fitness landscapes varied among environments separated by a few centimetres: interspecific competition selected for higher metabolism, and a faster pace‐of‐life, relative to competition‐free environments.
Populations experience a mosaic of competitive regimes; we find metabolism mediates a competition-colonization trade-off across these regimes. Although high metabolic phenotypes possess greater competitive ability, in the absence of competitors, low metabolic phenotypes are better colonizers.
Spatial heterogeneity and the variable selection on metabolic rates that it generates is likely to maintain variation in metabolic rate, despite strong selection in any single environment.
Pettersen AK, Hall MD, White CR, Marshall DJ (2020) Metabolic rate, context-dependent selection, and the competition-colonization trade-off. Evolution LettersPDFDOI
Lab life has, of necessity, been curtailed throughout the world but it has provided an opportunity for researchers to spend time trawling the literature for data to use in meta-analyses. Our lab is no different and so in this edition of lab life we aim to give an overview about what some of our members have been working on.
One tricky element of this data collection has been converting the different body size measurements to mass. For example, a paper may present data on head width in wasps, hind tibia length in grasshoppers, or carapace lengths in crabs but Michaela needs to convert this to a measure of mass. To do this she has had to create a repository of morphometric allometries for a number of different species, another great resource for other meta analyses that the lab group might do in which body mass needs to be calculated.
A separate study is also underway that is looking at not only the number of offspring but the size of those offspring in relation to maternal size. So, Melanie Lovass, with help from Michaela, has been compiling data to enable Hayley Cameron and our (now virtual) visitor Darren Johnson from California State University to ask the question: do bigger mothers produce bigger and/or more offspring and are there any differences between warm-blooded and cold-blooded animals?
PhD student, Emily Richardson is particularly interested in organisms that have complex life histories or, in other words, go through metamorphosis to become adults. Emily is gathering data on growth rates in amphibians, fish and marine invertebrates to test the theory that growth rate is maximised relative to mortality rate at the time of metamorphosis, which would mean that fitness is increased.
George Jarvis and Sam Ginther are also doing meta-analyses that relate to their PhD projects. George, like Emily, is interested in organisms with complex life histories but he is looking at large scale evolutionary change in metabolic rate. For his meta-analysis he is compiling metabolic data from marine invertebrates and looking at how metabolic rates vary between species with different developmental modes. With this work, he hopes to better understand the evolution of metabolic rate in complex life cycles.
Sam is interested in the cost of reproduction. He is collecting data on metabolic rates in reproductive and non-reproductive adults as well as their offspring. This will help him understand how the energy used for reproduction affects the production of offspring in species with dramatically different life histories.
Louise Noergaard has just started a post doc in the CGB and is busy working on a collaborative project between Dustin Marshall and Beth McGraw from Penn State University. Louise is looking at the relationship between wing length and body mass in mosquitoes and assessing how these measures of size relate to lifetime reproductive output. This information can then be put into a model that will consider how these measures of size and reproductive output affect existing predictions of mosquito spread.
Authors: Emily J Lombardi, Candice L Bywater, and Craig R White
Published in:Journal of Experimental Biology
The oxygen and capacity-limited thermal tolerance (OCLTT) hypothesis proposes that the thermal tolerance of an animal is shaped by its capacity to deliver oxygen in relation to oxygen demand. Studies testing this hypothesis have largely focused on measuring short-term performance responses in animals under acute exposure to critical thermal maximums. The OCLTT hypothesis, however, emphasises the importance of sustained animal performance over acute tolerance.
The present study tested the effect of chronic hypoxia and hyperoxia during development on moderate to long-term performance indicators at temperatures spanning the optimal temperature for growth in the speckled cockroach, Nauphoeta cinerea.
In contrast to the predictions of the OCLTT hypothesis, development under hypoxia did not significantly reduce growth rate or running performance, and development under hyperoxia did not significantly increase growth rate or running performance. The effects of developmental temperature and oxygen on tracheal morphology and metabolic rate were also not consistent with OCLTT predictions, suggesting that oxygen delivery capacity is not the primary driver shaping thermal tolerance in this species.
Collectively, these findings suggest that the OCLTT hypothesis does not explain moderate to long-term thermal performance in N. cinerea, which raises further questions about the generality of the hypothesis.
Lombardi EJ, Bywater CL, White CR (2020) The effect of ambient oxygen on the thermal performance of a cockroach, Nauphoeta cinerea. Journal of Experimental BiologyPDFDOI
We all know the expression “you are what you eat” but do juvenile diets determine your adult size or can adult diets change things? Gonçalo Poças, Alexander Crosbie and Christen Mirth have used a model organism – the fruit fly Drosophila melanogaster – to explore this question.
They found that while larval nutritional conditions play the dominant role in determining both adult body weight and appendage size in D. melanogaster, the adult diet can adjust body weight as the flies age.
The team manipulated both the quality and energy quantity of diets that larval and adult flies received in two experiments.
In the first experiment calorie content was held stable but the ratio of protein to carbohydrate was either 1:2 (high quality) or 1:10 (low quality). In the second experiment flies were fed a high quality diet but with two different calorie contents. The low calorie content diet was 25% of the high calorie diet.
Importantly, larvae were reared on one of the two diets in each experiment and then after “eclosing” (emerging as an adult) they either remained on the same diet or were switched to the alternative diet. This allowed the researchers to tease apart at what life-stage diet was most influential in determining measures of adult size.
Adult weights, wing area and femur size were measured at 3, 10 and 17 days old. The team predicted that adult weight would be more influenced by adult diet but that wing area and femur size would be determined by larval diet.
For the most part, the team found that the larval diet contributed more to differences in adult weight, wing area, and femur size in both males and females, but the quality of the larval diet had greater effects on adult size than the calorie content.
In addition to the effects of larval diet on adult size traits, Gonçalo and his colleagues found that animals subjected to poor larval nutrition were able to increase their body weight if maintained on good quality diets during the adult stages.
One surprising result the team uncovered was that the adult femur size changed with age, and depended on both the larval and adult diet. This suggests that the size of adult appendages might not be as fixed as previously thought.
So, to return to our initial question. Yes, the juvenile diet is very important in determining measures of adult size, but adult diets can make up for nutritional deficiencies in earlier life to some extent – in fruit flies at least.