The mitotic spindle must function in cell types that vary greatly in size, and its dimensions scale with the rapid, reductive cell divisions that accompany early stages of development. The mechanism responsible for this scaling is unclear, because uncoupling cell size from a developmental or cellular context has proven experimentally challenging. Here we combined microfluidic technology with Xenopus egg extracts to characterize spindle assembly within discrete, geometrically defined volumes of cytoplasm. Reductions in cytoplasmic volume, rather than developmental cues or changes in cell shape, were sufficient to recapitulate spindle scaling observed in Xenopus embryos. Thus, mechanisms extrinsic to the spindle, specifically a limiting pool of cytoplasmic component(s), play a major role in determining spindle size.

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We propose a new method for developing standard-weight (W s ) equations for use in the computation of relative weight (W r ) because the regression line-percentile (RLP) method often leads to length-related biases in W s equations. We studied the structural properties of W s equations developed by the RLP method through simulations, identified reasons for biases, and compared W s equations computed by the RLP method and the new method. The new method is similar to the RLP method but is based on means of measured weights rather than on means of weights predicted from regression models. The new method also models curvilinear W s relationships not accounted for by the RLP method. For some length-classes in some species, the relative weights computed from W s equations developed by the new method were more than 20 W r units different from those using W s equations developed by the RLP method. We recommend assessment of published W s equations developed by the RLP method for length-related bias and use of the new method for computing new W s equations when bias is identified.

Competitive ability changed across a range of 3-26°C among three fish species that show longitudinal replacement in Rocky Mountain streams: brook trout (Salvelinus fontinalis) at high elevations, brown trout (Salmo trutta) at middle elevations, and creek chub (Semotilus atromaculatus) at low elevations. Competitive ability was measured by food consumption and aggression in a stream tank. At 20°C, the trout species were competitively equal, and both were competitively superior to creek chub. Creek chub began to have competitive success against brook trout at 22°C and brown trout at 24°C, temperatures stressful but not lethal for the trout. Creek chub became competitively dominant over brook trout at 24°C and brown trout at 26°C, temperatures lethal to a portion of each trout species. We examined whether reduced food consumption was due to appetite loss or the presence of other species. For brook trout, interactions influenced feeding behavior at 22°C, but appetite loss became important at 24°C. For brown trout, interactions influenced feeding behavior at 24°C, but appetite loss became important at 26°C. For creek chub, there was an interaction between behavioral interactions and appetite in determining food consumption. Field data support a transition from trout to non-trout fishes at 22-25°C.

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