Mechanism of growth and sex determination in the Atlantic silverside, Menidia menidia, and the spatial scale of local adaptation
Atlantic silversides, Menidia menidia
For my dissertation research, I have chosen to use Menidia menidia (Atlantic silverside) as a model organism to understand the spatial scale of the sex determining mechanism and the mechanisms that contribute to growth rate and gonad development along a latitudinal gradient. Populations along this gradient are locally adapted and exhibit unique patterns in several quantitative traits including the form of sex determination, growth rate and vertebral number.
Determining the fine-scale pattern of the sex determining mechanism in M. menidia along a steep environmental cline.
Southern populations of M. menidia exhibit temperature-dependent sex determination (TSD) and northern populations exhibit genetic sex determination (GSD). The adaptive significance of temperature-dependent sex determination in M. menidia is well-known, but the spatial scale of changes in quantitative traits along its range is unknown. As part of a three-year study, we have collected embryos from 35 sites (~50 km apart) from FL to Nova Scotia and reared fish in a common garden to understand the underlying patterns of several traits. I have focused my interest in the mode of sex determination. Level of TSD exhibits a positive linear correlation with growing season over the species’ range.
Determining the sensitivity of cytochrome P450 aromatase to temperature in M. menidia populations along a latitudinal cline.
I compared the developmental pattern of aromatase expression in a population of M. menidia with both TSD and GSD. I reared larvae during the thermosensitive window at two temperatures in order to determine P450 arom (aromatase; gene involved in sex determination) expression prior to and during gonadal differentiation and development. As expected, aromatase was significantly elevated at feminizing temperatures only in the population with TSD. The GSD population displayed no differences in expression between the feminizing and masculinizing temperature, as expected. Results indicate that aromatase is responsive to temperature in fish with TSD, but the expression patterns (including timing and levels) are considerably different between populations. To our knowledge, this is the first study to address differences in expression of this gene in populations of a single species with two different forms of sex determination.
Are M. menidia populations with environmental sex determination more sensitive to exogenous estrogens than populations with predominantly genetic sex determination? Additionally, do wild populations along an urbanization gradient show evidence of endocrine disruption.
We used the gradient in sex determination in M. menidia, thus sensitivity to the environmental influence during gonad differentiation, to test for differential population susceptibility to a common municipal wastewater constituent, 17 β-estradiol (E 2).
I used four populations with varying levels of TSD and GSD. Populations with TSD responded with greater skews in sex ratio and had significantly altered sex ratios at lower concentrations than populations with predominantly GSD. Additionally, we wanted to assess the potential sensitivity of wild populations to estrogenic compounds. We measured the sex ratio in 12 M. menidia populations over a three-year period along a pronounced longitudinal gradient in sewage effluent and urbanization across Long Island. Sex ratios are significantly female-biased in western, more urbanized estuaries relative to eastern, relatively pristine estuaries. This relationship was significant for each of the three years of the study and is a unique pattern of this kind in a marine species.
Do constraints for gonad development exist among populations with different developmental patterns? To carry this out, we will assess the ontogeny of gonad differentiation among four populaitons with 1) slow growth, high TSD 2) moderate growth, moderate TSD 3) fast growth, complete GSD and 4) fast growth, moderate TSD.
Developing M. menidia gonad
Timing of gonad development appears to be delayed in fish from southern latitudes (which exhibit primarily TSD and slow growth) relative to fish from northern latitudes (GSD, fast growth). Therefore, I will compare the pattern of development (using histology) with a combination of populations that exhibit differences in intrinsic growth rate and mode of sex determination. I hypothesize that sex determination and differentiation is determined primarily by growth rate. Fish with faster growth rates will likely differentiate earlier and quicker, regardless of the mode of sex determination. This work is in progress.
Do differences in insulin-like growth factor-1 (IGF-1; a measureable proxy for growth rate) exist between fish selected for fast and slow growth? We will compare both artificially and naturally selected populations to see if IGF-1 responds differently under these scenarios.
Northern and southern populations of M. menidia exhibit strong differences in intrinsic growth rates and these differences are likely due to changes in the regulation of IGF-1. Additionally, a lab I am a part of has M. menidia populations that have been artificially selected to produce fish with fast, slow and intermediate growth. I have collected plasma from both naturally and artificially selected fish to measure circulating IGF-1. The goal of this work is to see if circulating IGF-1 differs between selection scenarios and if growth rate correlates with IGF-1 in this species.
