Sexual Reproduction in Flowering Plants Set-3

Q1. In a flowering plant with somatic chromosome number $2n=18$, calculate the chromosome numbers of (i) the egg cell, (ii) a male gamete (sperm), (iii) the zygote (embryo) and (iv) the primary endosperm nucleus after normal double fertilization.

Q2. A pistil has genotype $S_1S_2$ under gametophytic self-incompatibility (GSI). A pollen donor plant is heterozygous $S_2S_3$ and produces equal numbers of $S_2$ and $S_3$ pollen grains. Under GSI a pollen grain is rejected if its haploid $S$-allele matches either allele of the pistil. What fraction of pollen grains from the donor will be compatible on this pistil?

Q3. In a flowering plant with somatic chromosome number $2n=16$ (so $n=8$) the embryo sac is tetrasporic and the central cell contains four polar nuclei (each of ploidy $n$) before fertilization. After normal double fertilization (one sperm fuses with egg, the other with the central cell), the chromosome number of the primary endosperm nucleus will be:

Q4. In an angiosperm with somatic $2n=24$ ($n=12$) an unreduced pollen grain (due to meiotic failure) is $2n=24$ and releases two sperm nuclei each with $24$ chromosomes. This pollen fertilizes a normal ovule (egg $n$; central cell with two polar nuclei each $n$). Determine the chromosome numbers of (i) the embryo and (ii) the primary endosperm nucleus, and state whether the resulting seed is likely to be viable if normal viability requires embryo ploidy $2n$ and endosperm ploidy $3n$.

Q5. Species A has somatic chromosome number $2n=14$ ($n=7$) and species B has $2n=28$ ($n=14$). For normal endosperm development a maternal:paternal genomic ratio of $2:1$ (two maternal haploid contributions to one paternal haploid contribution) is generally required. Which cross direction between A and B will produce a primary endosperm with maternal:paternal genomic ratio $2:1$?

Q6. In a flower each anther has $4$ microsporangia. On average each microsporangium contains $120$ microsporocytes and each microsporocyte undergoes meiosis to produce $4$ microspores. If $25\%$ of the microspores fail to develop into viable pollen grains, how many viable pollen grains will a flower with $5$ stamens produce?

Q7. In a flowering plant the somatic chromosome number is $2n=14$. A meiotic failure produces an unreduced pollen grain that gives rise to male gametes with chromosome number $2n$. If one such $2n$ male gamete fertilises a normal egg ($n$) and the other $2n$ male gamete fuses with the two normal polar nuclei (each $n$), what will be the chromosome numbers of the embryo and of the primary endosperm nucleus respectively?

Q8. A diploid plant with somatic chromosome number $2n=18$ ($n=9$) is pollinated by a tetraploid plant ($4n=36$). The tetraploid produces reduced male gametes of $2n$ chromosomes (i.e., $2x$ gametes). Determine the chromosome number of the embryo and of the primary endosperm nucleus, state the maternal:paternal genome ratio in the endosperm, and indicate the most likely effect on seed development.

Q9. A stigma with S genotype $S_{1}S_{2}$ is pollinated by pollen from a plant with genotype $S_{2}S_{3}$. Assuming no dominance among S-alleles, under gametophytic self-incompatibility (GSI) pollen compatibility is determined by the haploid genotype of each pollen grain, while under sporophytic self-incompatibility (SSI) compatibility is determined by the diploid sporophyte genotype of the pollen parent. What fraction of the pollen grains from the donor will be compatible with the $S_{1}S_{2}$ stigma under GSI and under SSI respectively?

Q10. In a species with somatic chromosome number $2n=16$ a mutation causes the degeneration of one polar nucleus so that the central cell contains only a single polar nucleus ($n$). If normal fertilisation occurs — the egg ($n$) is fertilised by a normal male gamete ($n$), and the single polar nucleus fuses with the other normal male gamete ($n$) — what will be the chromosome numbers of the embryo and the primary endosperm nucleus, and what will be the maternal:paternal genome ratio in the endosperm?