During meiosis I, homologous chromosomes join together to form bivalents. Through trial and error, bivalents achieve stable bipolar orientations (attachments) on the spindle that eventually allow the segregation of homologous chromosomes to opposite poles. Bipolar orientations are stable through tension generated by poleward forces to opposite poles. Unipolar orientations lack tension and are stereotypically not stable. The behavior of sex chromosomes during meiosis I in the male black widow spider Latrodectus mactans (Araneae, Theridiidae) challenges the principles governing such a scenario. We found that male L. mactans has two distinct X chromosomes, X and X. The X chromosomes join together to form a connection that is present in prometaphase I but is lost during metaphase I, before the autosomes disjoin at anaphase I. We found that both X chromosomes form stable unipolar orientations to the same pole that assure their co-segregation at anaphase I. Using micromanipulation, immunofluorescence microscopy, and electron microscopy, we studied this unusual chromosome behavior to explain how it may fit the current dogma of chromosome distribution during cell division.
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A pproximately 11% of Americans (20% of men and 8% of women) are left-handed. Similar percentages of left-handers have been observed among medical personnel. The state of being left-hand dominant is perceived as a mere inconvenience by some; however, some lefthanded surgeons believe that they have inadequacy in the field. 1 Studies have shown that left-handed people are more prone to unintentional injuries, motor vehicle accidents, and head trauma. 1,2 These findings have been attributed to environmental barriers associated with living in a world that accommodates to right-handed individuals.In a survey by Adusumilli and colleagues, 2 the authors were surprised to learn that 1 in 10 left-handed surgeons reported difficulty in being treated by another left-handed surgeon. Although they did not ascertain the reasons for this, it is possible that the surgeons understood the challenges they had faced in their training and perceived left-handers as having an innate disadvantage in surgical skill. They noted that 50% of left-handed surgeons reported feeling anxious about their laterality and 1 in 4 was apprehensive about entering a surgical specialty.There is also a lack of mentoring and laterality-related guidance in medical school and residency. Surgeons surveyed by Adusumilli reported that only 1 in every 10 residency programs offered laterality-related mentoring to left-handed residents, and only 3% of these surgeons reported receiving laterality-related mentoring in medical school. 2 Other surveys have reported no mentoring regarding laterality at any point in training.Instrumentation often poses the greatest challenge for the left-handed surgeon. The first practical exposure a medical student has in surgery is handling accessory instruments, and it is rare that left-handed versions of these instruments are available. It is plausible that unpleasant initial tasks and difficulties with surgical skills might deter left-handed medical students from choosing a surgical career, although no data are available to associate this. 2 Many surgical instruments do not express laterality; however, some important ones do. For a piece of equipment designed for a right-handed user, the natural and often subconscious force applied to the instrument cannot be replicated by the left-hander without careful thought. An unnatural position must be assumed which creates inefficiency and potential error in execution. 3 Ordinarily, left-handed surgeons must reverse the usual motion to open right-handed needle holders, hemostats, and other locking instruments. Otherwise, the left-handed surgeon must attempt to learn to use these locking instruments with the right hand. Both methods could be fatiguing and unnatural for left-handed surgeons. Not only are left-handed instruments more expensive but they also are more difficult to find and often dated. According to the survey by Adusumilli and colleagues, 2 only 13% of residency programs offer left-handed instruments.
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