Nucleotide-dependent conformational change near the fulcrum region in Dictyostelium myosin II

In skeletal muscle myosin, the reactive thiols (SH1 and SH2) are close to a proposed fulcrum region that is thought to undergo a large conformational change. The reactive thiol region is thought to transmit the conformational changes induced by the actin–myosin–ATP interactions to the lever arm, which amplifies the power stroke. In skeletal muscle myosin, SH1 and SH2 can be chemically cross-linked in the presence of nucleotide, trapping the nucleotide in its pocket. Although the flexibility of the reactive thiol region has been well studied in skeletal muscle myosin, crystal structures of truncated nonmuscle myosin II from Dictyostelium in the presence of various ATP analogs do not show changes at the reactive thiol region that would be consistent with the SH1–SH2 cross-linking observed for muscle myosin. To examine the dynamics of the reactive thiol region in Dictyostelium myosin II, we have examined a modified myosin II that has cysteines at the muscle myosin SH1 and SH2 positions. This myosin is specifically cross-linked at SH1–SH2 by a chemical cross-linker in the presence of ADP, but not in its absence. Furthermore, the cross-linked species traps the nucleotide, as in the case of muscle myosin. Thus, the Dictyostelium myosin II shares the same dynamic behavior in the fulcrum region of the molecule as the skeletal muscle myosin. This result emphasizes the importance of nucleotide-dependent changes in this part of the molecule.

The Dynamic Behavior of Individual Microtubules Associated with Chromosomes In Vitro

Mitotic movements of chromosomes are usually coupled to the elongation and shortening of the microtubules to which they are bound. The lengths of kinetochore-associated microtubules change by incorporation or loss of tubulin subunits, principally at their chromosome-bound ends. We have reproduced aspects of this phenomenon in vitro, using a real-time assay that displays directly the movements of individual chromosome-associated microtubules as they elongate and shorten. Chromosomes isolated from cultured Chinese hamster ovary cells were adhered to coverslips and then allowed to bind labeled microtubules. In the presence of tubulin and GTP, these microtubules could grow at their chromosome-bound ends, causing the labeled segments to move away from the chromosomes, even in the absence of ATP. Sometimes a microtubule would switch to shortening, causing the direction of movement to change abruptly. The link between a microtubule and a chromosome was mechanically strong; 15 pN of tension was generally insufficient to detach a microtubule, even though it could add subunits at the kinetochore–microtubule junction. The behavior of the microtubules in vitro was regulated by the chromosomes to which they were bound; the frequency of transitions from polymerization to depolymerization was decreased, and the speed of depolymerization-coupled movement toward chromosomes was only one-fifth the rate of shortening for microtubules free in solution. Our results are consistent with a model in which each microtubule interacts with an increasing number of chromosome-associated binding sites as it approaches the kinetochore.

Induced intensification: Agricultural change in Bangladesh with implications for Malthus and Boserup

Bangladesh is dominated by a small-holder agrarian economy under extreme stress. Production shortfalls, increasing economic polarization, and chronic malnutrition are persistent, but major famine has been diverted in part by significant growth in agriculture. This recent history is open to both Malthusian and Boserupian interpretations—a history we explore here through a test of the induced intensification thesis of agricultural change. This thesis, framed by variations in the behavior of small-holders, has grown from a simple demand-production relationship to a consideration of the mediating influences on that relationship. The induced intensification thesis is reviewed and tested for 265 households in 6 villages in Bangladesh from 1950–1986. A time-series analysis of an induced intensification model provides relatively high levels of explained variance in cropping intensity (frequency and land productivity) and also indicates the relative impacts of household class, environment, and cropping strategies. On average, the small-holders in question kept pace with the demands on production, although important class and village variations were evident and the proportion of landless households increased. These results, coupled with evidence that agricultural growth involved intensification thresholds, provide clues about Malthusian and Boserupian interpretations of Bangladesh, and suggest that small-holder agriculture there is likely to continue on a “muted” path of growth.

Climate change is affecting altitudinal migrants and hibernating species

Calendar date of the beginning of the growing season at high altitude in the Colorado Rocky Mountains is variable but has not changed significantly over the past 25 years. This result differs from growing evidence from low altitudes that climate change is resulting in a longer growing season, earlier migrations, and earlier reproduction in a variety of taxa. At our study site, the beginning of the growing season is controlled by melting of the previous winter's snowpack. Despite a trend for warmer spring temperatures the average date of snowmelt has not changed, perhaps because of the trend for increased winter precipitation. This disjunction between phenology at low and high altitudes may create problems for species, such as many birds, that migrate over altitudinal gradients. We present data indicating that this already may be true for American robins, which are arriving 14 days earlier than they did in 1981; the interval between arrival date and the first date of bare ground has grown by 18 days. We also report evidence for an effect of climate change on hibernation behavior; yellow-bellied marmots are emerging 38 days earlier than 23 years ago, apparently in response to warmer spring air temperatures. Migrants and hibernators may experience problems as a consequence of these changes in phenology, which may be exacerbated if climate models are correct in their predictions of increased winter snowfall in our study area. The trends we report for earlier formation of permanent snowpack and for a longer period of snow cover also have implications for hibernating species.

Gregarious behavior in desert locusts is evoked by touching their back legs

Desert locusts in the solitarious phase were repeatedly touched on various body regions to identify the site of mechanosensory input that elicits the transition to gregarious phase behavior. The phase state of individual insects was measured after a 4-h period of localized mechanical stimulation, by using a behavioral assay based on multiple logistic regression analysis. A significant switch from solitarious to gregarious behavior occurred when the outer face of a hind femur had been stimulated, but mechanical stimulation of 10 other body regions did not result in significant behavioral change. We conclude that a primary cause of the switch in behavior that seeds the formation of locust swarms is individuals regularly touching others on the hind legs within populations that have become concentrated by the environment.

Rapid, local adaptation of zooplankton behavior to changes in predation pressure in the absence of neutral genetic changes

Organisms producing resting stages provide unique opportunities for reconstructing the genetic history of natural populations. Diapausing seeds and eggs often are preserved in large numbers, representing entire populations captured in an evolutionary inert state for decades and even centuries. Starting from a natural resting egg bank of the waterflea Daphnia, we compare the evolutionary rates of change in an adaptive quantitative trait with those in selectively neutral DNA markers, thus effectively testing whether the observed genetic changes in the quantitative trait are driven by natural selection. The population studied experienced variable and well documented levels of fish predation over the past 30 years and shows correlated genetic changes in phototactic behavior, a predator-avoidance trait that is related to diel vertical migration. The changes mainly involve an increased plasticity response upon exposure to predator kairomone, the direction of the changes being in agreement with the hypothesis of adaptive evolution. Genetic differentiation through time was an order of magnitude higher for the studied behavioral trait than for neutral markers (DNA microsatellites), providing strong evidence that natural selection was the driving force behind the observed, rapid, evolutionary changes.

Long-term change in the organization of inventive activity

Relying on a quantitative analysis of the patenting and assignment behavior of inventors, we highlight the evolution of institutions that encouraged trade in technology and a growing division of labor between those who invented new technologies and those who exploited them commercially over the nineteenth and early-twentieth centuries. At the heart of this change in the organization of inventive activity was a set of familiar developments which had significant consequences for the supply and demand of inventions. On the supply side, the growing complexity and capital intensity of technology raised the amount of human and physical capital required for effective invention, making it increasingly desirable for individuals involved in this activity to specialize. On the demand side, the growing competitiveness of product markets induced firms to purchase or otherwise obtain the rights to technologies developed by others. These increasing incentives to differentiate the task of invention from that of commercializing new technologies depended for their realization upon the development of markets and other types of organizational supports for trade in technology. The evidence suggests that the necessary institutions evolved first in those regions of the country where early patenting activity had already been concentrated. A self-reinforcing process whereby high rates of inventive activity encouraged the evolution of a market for technology, which in turn encouraged greater specialization and productivity at invention as individuals found it increasingly feasible to sell and license their discoveries, appears to have been operating. This market trade in technological information was an important contributor to the achievement of a high level of specialization at invention well before the rise of large-scale research laboratories in the twentieth century.