Govind C K
Life Sciences Division, University of Toronto, Scarborough, Ontario, Canada.
J Neurobiol. 1992 Dec;23(10):1423-45. doi: 10.1002/neu.480231006.
An enduring debate in the study of development is the relative contribution of genetic and epigenetic factors in the genesis of an organism, that is, the nature vs. nurture debate. The behavior of the paired claws in the lobster offers promising material for pursuing this debate because of the way they develop. The paired claws and their closer muscles are initially symmetrical; both are slender in appearance and have a mixture of fast and slow fibers in their closer muscles. During a critical period of development, they become determined into a major (crusher) and minor (cutter) claw and during subsequent development acquire their final form and behavior: The crusher becomes a stout, molar-toothed claw capable of closing only slowly because its closer muscle has 100% slow fibers while the cutter becomes a slender, incisor-toothed claw capable of closing rapidly because its closer muscle has 90% fast fibers. Our initial hypothesis was that the more active claw became the crusher and its less active counterpart the cutter. Presumably, nerve activity would influence muscle transformation, which in turn would influence the exoskeleton to which they attach and hence claw morphology. Curtailing nerve activity to the claw prevented crusher development, while reflex activation of a claw promoted its development; both results support the notion that nerve activity directly regulates claw form and function. This is not, however, the case, for when both claws were reflexly exercised neither formed a crusher, signifying rather that bilateral differences in predominantly mechanoreceptive input to the paired claws somehow lateralized the claw ganglion [central nervous system (CNS)] into a crusher and cutter side. The side experiencing the greater activity becomes the crusher side while the contralateral side becomes the cutter and is also inhibited from ever becoming a crusher. This initial lateralization in the CNS is expressed, via as yet unknown pathways, at the periphery in claw morphology, muscle composition, and behavior. The critical period defines a time when the CNS is susceptible to being lateralized into a crusher and cutter side. Such lateralization is dependent upon experience of the environment in the form of mechanoreceptive input. In the absence of such experience, the CNS is not lateralized and paired cutter claws develop. Thus, while the critical period for crusher determination is genetically determined the actual trigger is influenced by experience.
在发育研究中，一个长期存在的争论是遗传和表观遗传因素在生物体起源中的相对贡献，即先天与后天的争论。龙虾中一对爪子的行为为探讨这一争论提供了有前景的材料，因为它们的发育方式。这对爪子及其闭肌最初是对称的；外观上都很细长，闭肌中既有快肌纤维也有慢肌纤维。在发育的关键时期，它们分化为一个大爪（碾碎爪）和一个小爪（切割爪），并在随后的发育中获得最终的形态和行为：碾碎爪变成粗壮的、有臼齿状齿的爪子，只能缓慢闭合，因为其闭肌有100%的慢肌纤维，而切割爪变成细长的、有门牙状齿的爪子，能够快速闭合，因为其闭肌有90%的快肌纤维。我们最初的假设是，更活跃的爪子会变成碾碎爪，而不太活跃的对应爪子会变成切割爪。据推测，神经活动会影响肌肉转化，这反过来又会影响它们附着的外骨骼，从而影响爪子的形态。减少对爪子的神经活动会阻止碾碎爪的发育，而爪子的反射性激活会促进其发育；这两个结果都支持神经活动直接调节爪子形态和功能的观点。然而，情况并非如此，因为当两个爪子都进行反射性运动时，都不会形成碾碎爪，这表明对这对爪子的主要机械感受性输入的双侧差异以某种方式使爪神经节（中枢神经系统）分化为碾碎爪侧和切割爪侧。经历更多活动的一侧变成碾碎爪侧，而对侧变成切割爪侧，并且也被抑制永远不会变成碾碎爪。中枢神经系统中的这种最初分化通过尚未知晓的途径在爪子形态、肌肉组成和行为的外周表现出来。关键时期定义了中枢神经系统易分化为碾碎爪侧和切割爪侧的时间。这种分化取决于以机械感受性输入形式存在的环境体验。在没有这种体验的情况下，中枢神经系统不会分化，会发育出一对切割爪。因此，虽然碾碎爪确定的关键时期是由基因决定的，但实际的触发因素受体验的影响。
