# Understanding the mechanism of diversity generation through directional selection

> **NIH NIH F32** · STANFORD UNIVERSITY · 2024 · $78,892

## Abstract

PROJECT SUMMARY
 Understanding the general rules of adaptation has implications for treating diseases caused by evolving
entities such as cancer and drug-resistant infections. In attempts to uncover these general rules, previous
research has revealed a seemingly paradoxical phenomenon. On the one hand, under selective pressure,
adaptive mutations arise quickly, have large fitness effects, localize to a few functionally similar genes, and
likely exploit the same adaptive strategy. On the other hand, laboratory-evolved organisms show a wide range
of diverse physiological changes. This means that directional selection increases rather than decreases
phenotypic diversity. This phenomenon likely explains our limited ability to predict the effectiveness of
anticancer therapies, because even genetically similar cancers can have highly diverse physiologies. In this
project, I will use a collection of several hundred well-studied adaptive S. cerevisiae mutants, isolated from a
single evolution experiment, to test two mechanistic hypotheses of how selection might generate diversity. My
working hypothesis is that selection generates diversity through combinatorial loss of plastic responses. The
alternative hypothesis is that selection generates diversity through the gain of novel responses.
 To distinguish between these two hypotheses, I will measure the molecular phenotypes of the adaptive
mutants with two RNA sequencing technologies. In Aim 1, I will use bulk RNA sequencing on 10 carefully
selected mutants, grown in the environment in which they evolved, in order to gain the first insight into the
molecular mechanism of their adaptive strategy. In Aim 2, I will use a state-of-the-art, high-throughput,
barcode-aware RNA sequencing approach called Split-Seq to measure the full scope of the adaptive mutants'
phenotypes which are most prominent in various extreme environments. This will reveal if the adaptive mutants
achieve high fitness in the evolution environment and high diversity in other environments through loss of
plasticity or by creating novel responses. Uncovering the mechanism of how selection generates diversity will
contribute to our general understanding of adaptation and have implications for treating diseases driven by
evolutionary adaptation, such as cancer and drug-resistant infections.
 During my PhD, I received strong training in molecular genetics and learned basic wet lab skills but had
little exposure to evolutionary theory or genomics. Undertaking this project will greatly expand my conceptual
and technological toolkit, as I will learn molecular evolution and population genetics, as well as genomics
methods like barcode tracking and RNA sequencing. I will receive formal and informal training in evolutionary
theory, programming, genomics data analysis, grant writing, and professional leadership. This will prepare me
to become an independent researcher and leader in the interdisciplinary field of molecular and evolutionary
biology.

## Key facts

- **NIH application ID:** 10938613
- **Project number:** 5F32GM149046-02
- **Recipient organization:** STANFORD UNIVERSITY
- **Principal Investigator:** Alexandra Khristich
- **Activity code:** F32 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2024
- **Award amount:** $78,892
- **Award type:** 5
- **Project period:** 2023-08-14 → 2025-08-13

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10938613

## Citation

> US National Institutes of Health, RePORTER application 10938613, Understanding the mechanism of diversity generation through directional selection (5F32GM149046-02). Retrieved via AI Analytics 2026-05-26 from https://api.ai-analytics.org/grant/nih/10938613. Licensed CC0.

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