Why is Nature Able to Mold Some Phenotypes More Readily than Others? Investigating the Structure, Function and Evolution of ßeta-2 Tubulin in Drosophila Melanogaster

Date of Award

2018

Degree Name

M.S. in Biology

Department

Department of Biology

Advisor/Chair

Advisor: Mark Nielsen

Abstract

Some phenotypes have more ability to evolve than others, captured by the term “evolvability.” While some traits can evolve rapidly, such as the shape, color and size of a butterfly wing, the Drosophila testis specific beta-2 (ß2) tubulin protein, a fundamental component of the spermtail axonemes, has not evolved in over 60 million years. This protein is a main element of the microtubules within the axoneme which supports the motility of the sperm cell. There is a 9+2 configuration of microtubules, nine doublets of microtubules arranged along the outer edge of the structure with two central microtubules. Each microtubule consists of tubulin dimers of ß2 tubulin and the major alpha tubulin isoform 84B which is present in most cells of the body. Previous studies have shown that substitutions of the of beta-1 tubulin, a 95% identical paralog of beta-2 expressed in somatic cells, and chimeric tubulins composed of beta-1 and beta-2 tubulin sequence are unable to support a motile axoneme, indicating the axoneme is highly sensitive to beta tubulin structure. From these findings, evolutionary conservation and highly sensitive structure/ function relationship, two hypotheses tested here were developed for the long conservation of ß2 tubulin. The first, stabilizing selection: nature is constantly selecting a particular sequence even though other sequences may work due to differences in the quality of sperm produced. Or, it may be that there is no alternative sequences that function, and a co-evolutionary event with another protein found within the axoneme is required to release beta-2 tubulin to evolve. These hypotheses were tested using the substitution of a beta-2 ortholog, the gene in a different species which evolved from a common ancestor, was examined to determine its ability to produce a functional sperm in the Drosophila melanogaster model. If able to produce a functional sperm, stabilizing selection is supported; if unable, a co-evolutionary event has occurred. Through database searches the orthologous gene from the closest relation to Drosophila with a different amino acid sequence was Glossina morsitans morsitans (Tsetse fly). Glossina beta-2 was able to support a functional sperm in Drosophila melanogaster. The transgenic flies were fertile and able to produce progeny, TEM cross-sections of the spermtail revealed a 9+2 axoneme and the testes showed normal meiosis, spermhead shaping, and alignment, all beta-2 tubulin supported functions. These results suggest the protein is under stabilizing selection, another sequence available in nature is able to produce a functional product but for 60 million years, nature has been constantly selecting the same wildtype sequence. This indicates the opportunity to evolve for such alternative functional proteins may be rare and along a narrow path such as that which maintains beta-2 tubulin function.

Keywords

Biology, Evolution and Development, Developmental Biology, Beta-2 Tubulin, drosophila melanogaster, stabilizing selection, sperm, axoneme, microtubule, fruit fly, tsetse fly, glossina morsitans, testis, orthologue, evolution, evolutionary constraint, co-evolution

Rights Statement

Copyright © 2018, author

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