Hybridization

Adaptive Introgression:

Our new research on Saltmarsh and Nelson’s sparrows explores differential patterns of introgression on a genome-wide scale. Instead of focusing on regions of the genome that remain differentiated in the face of hybridization, we are instead focusing on regions that are exchanged preferentially between Saltmarsh and Nelson’s sparrows. Using whole-genome re-sequencing, we have identified candidate regions for adaptive introgression. These include genes related to tidal marsh adaptations that are introgressing preferentially from Saltmarsh Sparrows into hybrids.


 

Morphological Variation:

Detection of hybrids often relies on morphological characteristics. The use of phenotypic traits for hybrid identification broadly assumes that hybrids display intermediate characteristics compared to parental individuals. Yet, as a result of differential rates of introgression, hybrids may express a mosaic of parental traits, be indistinguishable from parental forms, or display extreme phenotypes compared to parental forms. These processes pose challenges for the identification of hybrid individuals based solely on morphology.

Boxplot of scores (range of values: 1-5) for the 13 individual plumage traits observed in 254 Saltmarsh, Nelson’s and hybrid individuals in this study, distributed across genotypic class: pure Nelson’s Sparrow (A), backcrossed in the direction of Nelson’s (B), backcrossed in the direction of Saltmarsh (C), and pure Saltmarsh Sparrow (D). R values are provided above each plot indicating the strength of the correlation for each plumage trait when regressed against the genetic data (hybrid index).
Boxplot of scores (range of values: 1-5) for the 13 individual plumage traits observed in 254 Saltmarsh, Nelson’s and hybrid individuals in this study, distributed across genotypic class.

To better understand the extent and role of morphological variation across hybrid zones, I am interested in quantifying rates of phenotypic introgression. I use a combination of approaches, including field based measurements and plumage scoring, digital photography, and image analysis. I am particularly interested in phenotype as a mechanisms for reproductive isolation and selection for darker plumage across tidal marsh gradients (salt marsh melanism).


 

Hybrid Zone Dynamics:

The focus of my dissertation research was understanding mechanisms responsible for shaping hybrid zone dynamics between Saltmarsh and Nelson’s sparrows. Specifically, how are species boundaries maintained in the face of ongoing gene flow and introgression. To address these questions, I combined both traditional field and genetic methods to test key predictions of the major models of hybrid zone evolution. Key areas of my dissertation research included:

  • Differential introgression and selection: Investigating rates of introgression for neutral and sex-linked markers and morphological traits.

    Curves show probability of occurrence for Saltmarsh Sparrows, Nelson's Sparrows, and hybrids - based on ecological niche models
    Curves show probability of occurrence for Saltmarsh Sparrows, Nelson’s Sparrows, and hybrids – based on ecological niche models
  • The role of habitat in shaping hybrid distributions: Combining field data with remote sensing and ecological niche modeling to test predictions of environment-dependent selection.
  • Hybrid fitness: Using data collected from three intensively monitored demographic sites to explore fitness differences in hybrids versus parental species. The demographic data allows for detailed exploration into the many aspects of reproductive success for both males and females (paternity, nest survival, nestling sex ratios, pairing success).

 

Conservation:

Both Saltmarsh and Nelson’s sparrows are species of high conservation priority in the northeast. The Saltmarsh Sparrow is considered globally vulnerable to extinction due to habitat loss/degradation and the impending impacts of sea-level rise. For this reason, the causes and outcomes of hybridization in this system are of interest to local and regional conservation efforts. Our goal is to provide a thorough evaluation of hybrid zone dynamics and aid in informing management decisions for these species.

Flow chart outlining considerations for decision making with respect to the conservation of saltmarsh-Nelson's sparrow hybrids.
Flow chart outlining considerations for decision making with respect to the conservation of saltmarsh-Nelson’s sparrow hybrids.