ONTOGENY OF THE PECTORALIS-MUSCLE IN THE LITTLE BROWN BAT, MYOTIS-LUCIFUGUS

The ontogeny of a primary flight muscle, the pectoralis, in the little brown bat (Myotis lucifugus: Vespertilionidae) was studied using hist ochemical, immunocytochemical, and electrophoretic techniques. In feta l and early neonatal (postnatal age 1-6 days) Myotis, histochemical te chniques for myofibrillar ATPase (mATPase) and antibodies for slow and fast myosins demonstrated the presence of two fiber types, here calle d types I and IIa. These data correlated with multiple transitional my osin heavy chain isoforms and native myosin isoforms demonstrated with SDS-PAGE and 4% pyrophosphate PAGE. There was a decrease in the distr ibution and number of type I fibers with increasing postnatal age. At postnatal age 8-9 days, the adult phenotype was observed with regard t o muscle fiber type (100% type IIa fibers) and myosin isoform profile (single adult MHC and native myosin isoforms). This ''adult'' fiber ty pe profile and myosin isoform composition preceeded adult function by about 2 weeks. For example, little brown bats were incapable of sustai ned flight until approximately postnatal day 24, and myofiber size did not achieve adult size until approximately postnatal day 25. Although Myotis pectoralis is unique in being composed of 100% type IIa fibers , transitional fiber types and isoforms were present. These transition al forms had been observed previously in other mammals bearing mixed a dult muscle fibers and which undergo transitional stages in muscle ont ogeny. However, in Myotis pectoralis, this transition transpires relat ively early in development. (C) 1994 Wiley-Liss, Inc.