Doctoral Dissertations

Date of Award

12-1990

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Zoology

Major Professor

David A. Etnier

Committee Members

Dewey L. Bunting, Arthur C. Echternacht, A. Murray Evans

Abstract

Populations of Etheostoma nigrum. the johnny darter, were examined to determine the presence or absence of geographic variation in meristic, morphometric, and karyotypic characters. These populations were from primarily south of the Great Lakes and west of the Appalachian Mountains. Meaningful geographic variation was found in the following meristic characters: dorsal soft rays, pectoral rays, supratemporal canal pores, , infraorbital canal pores, preoperculomandibular canal pores, squamation, and branchiostegal rays. While morphometric analysis demonstrated that the sexes of E. nigrum are dimorphic, with the male having longer fins and the female having a deeper body, measurements were not very useful for the separation of E. nigrum populations. Karyotypic analysis of E. nigrum populations demonstrated geographic variation in the number of types of chromosomes present and in fundamental arm number. Southern populations of E. nigrum had a fundamental arm number of 90, while northern E. nigrum had a fundamental arm number of 86. Etheostoma olmstedi, the presumed sister species of E. nigrum, was karyotyped and also had a fundamental arm number of 86. Two races of E. nigrum are recognized. One race is found in the Bayou Pierre, Homochitto, Big Black, Yazoo, and Obion river systems of Mississippi, Tennessee, and Kentucky. This race differs from other southern populations in having a higher number of dorsal soft rays (modally 13 compared with 12 in other southern populations), a more complete infraorbital canal system (modally 10, 11, or 12 pores compared with 8 or 9 pores in other southern populations), and a more complete supratemporal canal system (with 89% complete compared with 0-64% complete in other southern populations). A second race of E. nigrum is recognized from the Ohio River drainage. This race has fewer branchiostegal rays (modally 10 or 11 compared with 12 elsewhere) and fewer pectoral rays (modally 11 rays compared with 12 elsewhere). The variation possessed by E. nigrum populations provides important biogeographical evidence for several stream captures and past river associations. A hypothesis for the evolutionary history of E. nigrum and other members of the subgenus Boleosoma is presented.

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