A Comparison of Chorions from Eggs of Northern and Southern Populations of Fundulus heteroclitus

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Abstract
A comparative study of the chorions of eggs of northern and southern populations of F. heteroclitus by scanning and transmission electron microscopy reveals striking differences. Chorionic fibrils of eggs of the northern (Woods Hole [Massachusetts, USA]) population are very long, .apprx. 1.5 .mu. in diameter, and very sparsely distributed; the chorionic surface between attached fibrils is dotted with small protuberances. Most fibrils of the eggs of a southern (South Carolina) population are shorter, .apprx. 0.5 .mu. in diameter, and very densely distributed. The South Carolina eggs have a few longer and thicker (.apprx. 1.0 .mu.) fibrils in the vicinity of the micropyle. The fibrils of the Woods Hole eggs are club-shaped at their bases, surrounded by a collar of jelly at their attachment points, and are seated in an indentation in the chorion. Those of the South Carolina eggs show no such basal modifications and appear to extend from a small chorionic hillock. A surface coat of jelly is present on the ovulated eggs of both populations but appears to be thicker and denser on the eggs of southern origin. Scanning electron microscopy of freeze-fractured preparations of ovarian tissue from the 2 populations shows that the chorionic fibrils are present and attached to the developing chorion as soon as it is visible. Jelly is not present on the surface of the unovulated eggs. The data are discussed from the standpoint of considerations of the taxonomy and distribution of the species, and questions are raised concerning the possible significance of the structural differences observed.

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