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  1. The classic roles of mitochondria in energy production, metabolism, and apoptosis have been well defined. However, a growing body of evidence suggests that mitochondria are also active players in regulating stem cell fate decision and lineage commitment via signaling transduction, protein modification, and epigenetic modulations. This is particularly interesting for spermatogenesis, during which germ cells demonstrate changing metabolic requirements across various stages of development. It is increasingly recognized that proper male fertility depends on exquisitely controlled plasticity of mitochondrial features, activities, and functional states. The unique role of mitochondria in germ cell ncRNA processing further adds another layer of complexity to mitochondrial regulation during spermatogenesis. In this review, we will discuss potential regulatory mechanisms of how mitochondria swiftly reshape their features, activities, and functions to support critical germ cell fate transitions during spermatogenesis. In addition, we will also review recent findings of how mitochondrial regulators coordinate with germline proteins to participate in germ cell-specific activities. 
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  2. This study examined how humans spontaneously merge a sequence of discrete actions into a rhythmic pattern, even when periodicity is not required. Two experiments used a virtual throwing task, in which subjects performed a long sequence of discrete throwing movements, aiming to hit a virtual target. In experiment 1, subjects performed the task for 11 sessions. Although there was no instruction to perform rhythmically, the variability of the interthrow intervals decreased to a level comparable to that of synchronizing with a metronome; furthermore, dwell times shortened or even disappeared with practice. Floquet multipliers and decreasing variability of the arm trajectories estimated in state space indicated an increasing degree of dynamic stability. Subjects who achieved a higher level of periodicity and stability also displayed higher accuracy in the throwing task. To directly test whether rhythmicity affected performance, experiment 2 disrupted the evolving continuity and periodicity by enforcing a pause between successive throws. This discrete group performed significantly worse and with higher variability in their arm trajectories than the self-paced group. These findings are discussed in the context of previous neuroimaging results showing that rhythmic movements involve significantly fewer cortical and subcortical activations than discrete movements and therefore may pose a computationally more parsimonious solution. Such emerging stable rhythms in neuromotor subsystems may serve as building blocks or dynamic primitives for complex actions. The tendency for humans to spontaneously fall into a rhythm in voluntary movements is consistent with the ubiquity of rhythms at all levels of the physiological system. NEW & NOTEWORTHY When performing a series of throws to hit a target, humans spontaneously merged successive actions into a continuous approximately periodic pattern. The degree of rhythmicity and stability correlated with hitting accuracy. Enforcing irregular pauses between throws to disrupt the rhythm deteriorated performance. Stable rhythmic patterns may simplify control of movement and serve as dynamic primitives for more complex actions. This observation reveals that biological systems tend to exhibit rhythmic behavior consistent with a plethora of physiological processes. 
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