B chromosomes are enigmatic elements in thousands of plant and animal genomes that persist in populations despite being nonessential. They circumvent the laws of Mendelian inheritance but the molecular mechanisms underlying this behavior remain unknown. Here we present the sequence, annotation, and analysis of the maize B chromosome providing insight into its drive mechanism. The sequence assembly reveals detailed locations of the elements involved with the cis and trans functions of its drive mechanism, consisting of nondisjunction at the second pollen mitosis and preferential fertilization of the egg by the B-containing sperm. We identified 758 protein-coding genes in 125.9 Mb of B chromosome sequence, of which at least 88 are expressed. Our results demonstrate that transposable elements in the B chromosome are shared with the standard A chromosome set but multiple lines of evidence fail to detect a syntenic genic region in the A chromosomes, suggesting a distant origin. The current gene content is a result of continuous transfer from the A chromosomal complement over an extended evolutionary time with subsequent degradation but with selection for maintenance of this nonvital chromosome.

Demonstrated here is a detailed protocol for Agrobacterium-mediated genetic transformation of maize inbred lines using morphogenic genes Baby boom (Bbm) and Wuschel2 (Wus2). Bbm is regulated by the maize phospholipid transferase gene (Pltp) promoter, and Wus2 is under the control of a maize auxin-inducible (Axig1) promoter. An Agrobacterium strain carrying these morphogenic genes on transfer DNA (T-DNA) and extra copies of Agrobacterium virulence (vir) genes are used to infect maize immature embryo explants. Somatic embryos form on the scutella of infected embryos and can be selected by herbicide resistance and germinated into plants. A heat-activated cre/loxP recombination system built into the DNA construct allows for removal of morphogenic genes from the maize genome during an early stage of the transformation process. Transformation frequencies of approximately 14%, 4%, and 4% (numbers of independent transgenic events per 100 infected embryos) can be achieved for W22, B73, and Mo17, respectively, using this protocol.

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Two lines of Zea mays were developed as a short-generation model for maize. The Fast-Flowering Mini-Maize (FFMM) lines A and B are robust inbred lines with a significantly shorter generation time, much smaller stature, and better greenhouse adaptation than traditional maize varieties. Five generations a year are typical. FFMM is the result of a modified double-cross hybrid between four fast-flowering lines: Neuffer's Early ACR (full color), Alexander's Early Early Synthetic, Tom Thumb Popcorn, and Gaspe Flint, followed by selection for early flowering and desirable morphology throughout an 11-generation selfing regime. Lines A and B were derived from different progeny of the initial hybrid, and crosses between Mini-Maize A and B exhibit heterosis. The ancestry of each genomic region of Mini-Maize A and B was inferred from the four founder populations using genotyping by sequencing. Other genetic and genomic tools for these lines include karyotypes for both lines A and B, kernel genetic markers y1 (white endosperm) and R1-scm2 (purple endosperm and embryo) introgressed into Mini-Maize A, and 243 whole-genome resequencing data for Mini-Maize A.KEYWORDS maize; Zea mays; flowering time; heterosis; model system M ODEL genetic organisms are typically selected for their short generation time, small size, large number of progeny, and inexpensive maintenance. Maize began its use for genetic analysis before the concept of model organisms emerged. Nevertheless, it has continued to contribute to genetic studies despite its relatively longer generation time (typically two per year) and large stature because of the ease of conducting crosses due to its monoecious nature, large number of progeny, relatively large chromosomes, a suite of genetic markers visible at the kernel stage, several tools to readily manipulate genome or chromosomal dosage, and enormous natural variability (Nannas and Dawe 2015). Flowering time in maize landraces can vary from 2 to 11 months, and height can vary from 1 to 7 m (Kuleshov 1933), opening the possibility of selecting for small, fastflowering varieties more suitable as model genetic systems. Previous studies have generated short-generation varieties but none are available that maximize useful model organism characteristics. Here we describe the development of FastFlowering Mini-Maize (FFMM), which consists of two inbred lines, FFMM-A and FFMM-B (Figure 1). FFMM has many characteristics desirable in a model system, in that it can go from seed to seed in 60 days, produces many progeny, grows to ,1 m tall, and performs well under greenhouse conditions, while still being relevant to one of the most economically important cereal crops in the world.The goal of FFMM is to provide an open-source tool for researchers to reduce the timeline of research projects and improve greenhouse capacity. FFMM successfully decreases generation time and space requirements of maize. Five generations per year are routine. The kernel genetic markers, short generation time, and small stature also have potential ...

Plant minichromosomes have the potential for stacking multiple traits on a separate entity from the remainder of the genome. Transgenes carried on an independent chromosome would facilitate conferring many new properties to plants and using minichromosomes as genetic tools. The favored method for producing plant minichromosomes is telomere-mediated chromosomal truncation because the epigenetic nature of centromere function prevents using centromere sequences to confer the ability to organize a kinetochore when reintroduced into plant cells. Because haploid induction procedures are not always complete in eliminating one parental genome, chromosomes from the inducer lines are often present in plants that are otherwise haploid. This fact suggests that minichromosomes could be combined with doubled haploid breeding to transfer stacked traits more easily to multiple lines and to use minichromosomes for massive scale genome editing.