The supernumerary B chromosome of maize: drive and genomic conflict

The supernumerary B chromosome of maize is dispensable, containing no vital genes, and thus is variable in number and presence in lines of maize. In order to be maintained in populations, it has a drive mechanism consisting of nondisjunction at the pollen mitosis that produces the two sperm cells, and then the sperm with the two B chromosomes has a preference for fertilizing the egg as opposed to the central cell in the process of double fertilization. The sequence of the B chromosome coupled with B chromosomal aberrations has localized features involved with nondisjunction and preferential fertilization, which are present at the centromeric region. The predicted genes from the sequence have paralogues dispersed across all A chromosomes and have widely different divergence times suggesting that they have transposed to the B chromosome over evolutionary time followed by degradation or have been co-opted for the selfish functions of the supernumerary chromosome.

Sequence of the supernumerary B chromosome of maize provides insight into its drive mechanism and evolution

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.

Locating a site on the maize B chromosome that controls preferential fertilization

In maize, the B chromosome can undergo nondisjunction at the second pollen mitosis, producing sperm with two B chromosomes and sperm with zero B chromosomes. Preferential fertilization is the ability of the sperm carrying two B chromosomes to transmit more frequently to the embryo of a kernel than the sperm lacking the B chromosome. A translocation involving the B chromosome and chromosome 9, TB-9Sb, has been used to study preferential fertilization. The B-9 chromosome has the same properties of nondisjunction and preferential fertilization as the standard B chromosome. Deletion derivatives of B-9, which lack the centric heterochromatin and possibly some adjacent euchromatin, were tested for their ability to induce preferential fertilization. They were found to lack the capacity for preferential fertilization.

Maternal diet composition alters serum steroid and free fatty acid concentrations and vaginal pH in mice

We examined the effects of three maternal diets (very high fat (VHF), low fat (LF), and control (Purina 5015)) on serum steroids, free fatty acids (FFA), and vaginal pH in National Institutes of Health Swiss mice. Females were fed (VHF, n = 33; LF, n = 33; 5015, n = 48) from 4 to 16 weeks of age. Following breeding, female serum was collected at 0.5 (pre-implantation, early diestrus) or 8.5 (post-implantation, mid-diestrus) days post-coitus (dpc). The serum concentrations of 17β-estradiol, testosterone, progesterone, and FFA were analyzed at both collection points, and vaginal pH at 0.5 dpc. Striking differences in steroids and FFA were observed at 0.5 dpc among the groups. Estradiol was higher in the VHF (14.1 ± 3.0 pg/ml), compared with LF mice (5.2 ± 2.3 pg/ml; P≤ 0.05). In contrast, 0.5 dpc testosterone was lower in the VHF (10.5 ± 3.0 pg/ml) versus the LF group (32.7 ± 8.4 pg/ml; P≤ 0.05). At 8.5 dpc, progesterone was higher in the VHF (89.6 ± 6.7 ng/ml) versus the 5015 group (60.1 ± 4.9 ng/ml; P≤ 0.05). VHF mice had higher FFA concentrations at 0.5 dpc (1.0 ± 0.2 mmol/l) than LF and control mice (0.5 ± 0.1 and 0.6 ± 0.1 mmol/l respectively; P≤ 0.05). At 8.5 dpc, VHF females had higher serum FFA (0.8 ± 0.1 mmol/l) than LF and control females (0.4 ± 0.1 and 0.6 ± 0.1 mmol/l; P≤ 0.05). Mean vaginal pH of VHF females (6.41 ± 0.09) was lower than 5015 females (6.76 ± 0.10; P≤ 0.05). These diet-induced alterations in serum steroid and FFA concentrations might affect several reproductive processes, including preferential fertilization by one class of sperm over the other and sex bias in pre- and post-implantational embryonic development.

Microtubule-organelle associations during generative cell polarization in plumbago zeylanica

Sexual reproduction in the flowering plant Plumbago zeylanica is characterized by pollen in which the two sperm cells differ significantly with respect to size, shape and organellar content. The sperm cell with the majority of mitochondria (ave. 256) typically fuses with the central cell to form the nutritive endosperm in which the embryo will develop. The sperm cell containing essentially all of the numerous plastids (ave. 24) and few mitochondria (ave. 40) fuses with the egg to form the embryo. This system of preferential fertilization results in each sperm cell having a predetermined fate upon arrival at the embryo sac. Features determining the distribution of organelles in the sperm cells originate in the polarity of their precursor cell, the generative cell.

A study of the theoretical expectation ofF1hybridity resulting from bulk interpopulation hybridization in herbage grasses

SUMMARYA study of the theoretical expectation ofF1hybridity resulting from bulk interpopulation hybridization in herbage grasses has been made. It is concluded that the main factor affecting the percentage interpopulationalF1hybrid content of the seed product of a two-population mixed sward is degree of coincidence of population flowering patterns. The effects of differences in population parameters are examined for a range of population types. Other factors such as differences in total gamete frequencies, preferential fertilization and differential pre- and post-harvest seed losses are also considered.