Hormonal control of swimbladder sonic muscle dimorphism in the Lusitanian toadfish Halobatrachus didactylus.

Teresa Modesto,Adelino V M CanáRio

doi:10.1242/jeb.00581

Abstract

The swimbladder and associated sonic muscle of the Lusitanian toadfish Halobatrachus didactylus increase in size throughout life and are, respectively, 25% and 30% larger in type I (nest-holder) males than females, which may generate sexual differences in sound production. Sexual dimorphism in swimbladder is also evident in the morphological features of sonic muscle fibers. During the breeding season, type I males have smaller myofibril contracting zones surrounded by larger sarcoplasm areas compared with females, possibly an adaptation to speed and fatigue resistance for the production of long mating calls. Type II (floater) males show characteristics that are intermediate, but statistically not significantly different, between type I males and females. Six weeks after castration and androgen (testosterone and 11-ketotestosterone) replacement in type I and type II males there were no alterations either in swimbladder mass or fiber morphology. However, 17beta-estradiol induced a significant decrease in swimbladder mass and sarcoplasm area/myofibril area ratio. Six months after castration there was a clear reduction in the seasonal swimbladder hypertrophy in males and induction of sonic fiber morphological characteristics that resemble those occurring in females (low sarcoplasm area/myofibril area ratio). These results suggest that testicular factors are required to initiate sonic muscle hypertrophy and type I sonic fiber phenotype in H. didactylus, but a specific involvement of androgens has not been completely clarified.

Highlights

Toadfishes and midshipmen produce sounds associated with courtship and agonistic interactions
Sonic fibers have a number of morphological and biochemical adaptations for rapid contraction speed: (1) an unusual radial morphology [fibers are of polygonal shape and contain a core of sarcoplasm surrounded by a contractile cylinder of alternating slender ribbons of sarcoplasmic reticulum (SR) and myofibrils; Fawcett and Revel, 1961; Fine et al, 1993; Loesser et al, 1997], (2) an extremely well-developed SR forming about a third of the fiber volume (Appelt et al, 1991; FranziniArmstrong and Nunzi, 1983) and (3) the calcium transient is the fastest ever recorded in a vertebrate muscle (Rome et al, 1996, 1999)
The present study investigated whether H. didactylus sonic muscle size and/or sonic muscle fiber morphology are under androgenic control

Summary

Introduction

Toadfishes and midshipmen produce sounds associated with courtship and agonistic interactions. Sonic fibers have a number of morphological and biochemical adaptations for rapid contraction speed: (1) an unusual radial morphology [fibers are of polygonal shape and contain a core of sarcoplasm surrounded by a contractile cylinder of alternating slender ribbons of sarcoplasmic reticulum (SR) and myofibrils; Fawcett and Revel, 1961; Fine et al, 1993; Loesser et al, 1997], (2) an extremely well-developed SR forming about a third of the fiber volume (Appelt et al, 1991; FranziniArmstrong and Nunzi, 1983) and (3) the calcium transient is the fastest ever recorded in a vertebrate muscle (Rome et al, 1996, 1999). The functional design of a radial arrangement of alternative ribbons of SR and myofibrils minimises the distance and the transport time of calcium to the contractile proteins and back to the SR (Fine et al, 1993)

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