On the clock: Circadian rhythm gene expression in Oncorhynchus mykiss hatchlings derived from different life history cross types

Julie M Scardina, Purdue University

Abstract

The circadian system has been associated with every part of the life cycle including development rate, behavior, seasonal reproduction, and migration. This study focuses on the oscillatory molecular genetic components of the circadian system in Oncorhynchus mykiss, a fish species that matures into two different life history types, a resident form referred to as rainbow trout and a migratory (anadromous) form referred to as steelhead. We tested the hypothesis that these forms have begun to differentiate as early as the hatch stage by measuring gene expression of the molecular components of the circadian system - collectively referred to as clock genes - including: brain and muscle aryl hydrocarbon receptor nuclear translocator ( bmal), circadian locomotor output cycles kaput (clock), period (per), and cryptochrome (cry). Two bmal, three clock, four cry, and three per expressed sequence tags (ESTs) in O. mykiss were mined from public databases for a total of 12 candidate clock genes. These were sequenced, genotyped, and added to an existing linkage map to determine the number of copies present. All except two cry ESTs mapped to single positions, leaving 11 candidates for gene expression analysis. To evaluate whether progeny from these two different life history types display variation in absolute expression or pattern of expression of the circadian genes we created crosses between wild caught anadromous (A) and resident (R) O. mykiss forming three cross types (AxA, AxR, and RxR), collected the resulting embryos every 4 hours for a period of 24 hours at hatch, and measured gene expression. In a mixed model analysis, sampling time point was significant for bmal1, bmal2, clock1a , clock1b, clock3, cry1b , cryDASH, per1, and one copy of per4 (per4.1). This result was expected because circadian genes are known to cycle over time. The interaction between cross type and time point was significant for clock1b and cry1b indicating that the pattern of expression between crosses was different over time. No factors were significant for a second copy of per4 (per4.2). The five positive regulators (bmal and clock) showed similar expression profiles over time while the seven negative regulators (cry and per) were dissimilar. An analysis of periodicity suggested that the progeny of the resident by resident (RxR) cross fit the expected 24 hour period while the anadromous by anadromous (AxA) and anadromous by resident (AxR) progeny appeared to be arrhythmic. However, these differences could not be fully tested because the collection time frame was limited to 24 hours

Degree

M.S.

Advisors

Nichols, Purdue University.

Subject Area

Genetics|Zoology

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