McKim Laboratory

Dr. Kim S. McKim is investigating Meiosis, a process that is essential for sexual reproduction, and is using these fruit flies as a model organism to help us better understand Human Meiosis.

Research Summary

Meiosis is the process by which the chromosome number is divided precisely in half. When defects occur in the meiotic process the oocyte or sperm receives an abnormal number of chromosomes (aneuploidy). Aneuploidy is usually catastrophic and is the leading cause of infertility in women and the cause of disorders such as Down’s syndrome. Research in my laboratory is directed towards understanding meiosis in the model organism Drosophila melanogaster. By utilizing the experimental benefits of Drosophila, mutations that disrupt various steps in the meiotic program can be isolated and characterized. Currently, the lab focuses on two of the most important aspects of meiosis: i) the repair of programmed double strand breaks (DSB) in the DNA into crossovers, and ii) the involvement of crossovers in the segregation of homologous chromosomes. Since crossovers physically link homologous chromosomes together on a meiotic spindle, they direct segregation and ensure the precise splitting of the diploid genetic content into the gametes.

In the past we have taken a classical genetics approach to dissect the different steps of meiotic recombination and chromosome segregation; identifying genes using unbiased screens for recessive mutants causing nondisjunction of the sex chromosomes. More recently, we are using a variety of reverse genetics approaches such as gene targeting and transposon mobilization to mutate interesting genes identified based on their sequence. Most exciting is the development of techniques which allow efficient RNAi in the Drosophila germline. This allows us to knock down and usually eliminate the protein of almost any gene and study its affects on meiosis. This is particularly important for genes which are essential, since mutations in these genes cause lethality and thus meiosis, which must be studied in adult females, cannot be analyzed. With these new methods, the RNAi can be expressed only in the female germline, bypassing the requirement for a gene on viability. Finally, our work heavily depends on high resolution microscopy using immunofluorescence to detect important meiotic proteins. Antibodies to specific proteins are used in conjunction with high resolution imaging to investigate the structure and behavior of meiotic and mitotic chromosomes.

Dr. Kim S. McKim

Principal Investigator

Recent Publications

Joyce, EF, Paul A, Chen KE, McKim KS. 2012. Multiple Barriers to Non-homologous DNA End Joining During Meiosis in Drosophila. Genetics. 191:739-46. AbstractWebsite

Repair of meiotic double-strand breaks (DSBs) uses the homolog and recombination to yield crossovers while alternative pathways such as nonhomologous end-joining (NHEJ) are suppressed. Our results indicate that NHEJ is blocked at two steps of DSB repair during meiotic prophase: first by the activity of the MCM-like protein MEI-218 that is required for crossover formation and, second, by Rad51-related proteins SPN-B (XRCC3) and SPN-D (RAD51C) that physically interact and promote homologous recombination. We further show that the MCM-like proteins also promote the activity of the DSB repair checkpoint pathway, indicating an early requirement for these proteins in DSB processing. We propose that when a meiotic DSB is formed in the absence of both MEI-218 and SPN-B or SPN-D, a DSB substrate is generated that can enter the NHEJ repair pathway. Indeed, due to its high error rate, multiple barriers may have evolved to prevent NHEJ activity during meiosis.

Radford, SJ, Jang JK, McKim KS. 2012. The Chromosomal Passenger Complex is required for Meiotic Acentrosomal Spindle Assembly and Chromosome Bi-orientation. Genetics. 192:417-429. AbstractWebsite

During meiosis in the females of many species, spindle assembly occurs in the absence of the microtubule-organizing centers called centrosomes. In the absence of centrosomes, the nature of the chromosome-based signal that recruits microtubules to promote spindle assembly as well as how spindle bipolarity is established and the chromosomes orient correctly towards the poles is not known. To address these questions, we focused on the chromosomal passenger complex (CPC). We have found that the CPC localizes in a ring around the meiotic chromosomes that is aligned with the axis of the spindle at all stages. Using new methods which dramatically increase the effectiveness of RNAi in the germline, we show that the CPC interacts with Drosophila oocyte chromosomes and is required for the assembly of spindle microtubules. Furthermore, chromosome bi-orientation and the localization of the central spindle kinesin-6 protein Subito, which is required for spindle bipolarity, depend on the CPC components Aurora B and Incenp. Based on these data we propose that the ring of CPC around the chromosomes regulates multiple aspects of meiotic cell division including spindle assembly, the establishment of bipolarity, the recruitment of important spindle organization factors, and the bi-orientation of homologous chromosomes.

Radford, SJ, Harrison AM, McKim KS. 2012. Microtubule-depolymerizing Kinesin KLP10A Restricts the Length of the Acentrosomal Meiotic Spindle in Drosophila Females. Genetics. 192:431-440. AbstractWebsite

During cell division, a bipolar array of microtubules forms the spindle through which the forces required for chromosome segregation are transmitted. Interestingly, the spindle as a whole is stable enough to support these forces even though it is composed of dynamic microtubules, which are constantly undergoing periods of growth and shrinkage. Indeed, the regulation of microtubule dynamics is essential to the integrity and function of the spindle. We show here that a member of an important class of microtubule-depolymerizing kinesins, KLP10A, is required for the proper organization of the acentrosomal meiotic spindle in Drosophila melanogaster oocytes. In the absence of KLP10A, microtubule length is not controlled, resulting in extraordinarily long and disorganized spindles. In addition, the interactions between chromosomes and spindle microtubules are disturbed and can result in the loss of contact. These results indicate that the regulation of microtubule dynamics through KLP10A plays a critical role in restricting the length and maintaining bipolarity of the acentrosomal meiotic spindle and in promoting the contacts that the chromosomes make with microtubules required for meiosis I segregation.

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