Question

A species of cereal rye (Secale cereale) has a chromosome number of \(14,\) while a species of Canadian wild rye (Elymus canadensis) has a chromosome number of 28. Sterile hybrids can be produced by crossing Secale with Elymus. (a) What would be the expected chromosome number in the somatic cells of the hybrids? (b) Given that none of the chromosomes pair at meiosis I in the sterile hybrid (Hang and Franckowlak, 1984 ), speculate on the anaphase I separation patterns of these chromosomes.

Short answer

Answer: The expected chromosome number in the somatic cells of the hybrids is 42. The separation patterns of these chromosomes during anaphase I would be random and independent, with no pairing during meiosis I.

Step-by-step solution

(Step 1: Determine the chromosome number in the somatic cells of the hybrids)

To determine the expected chromosome number in the somatic cells of the hybrids, we need to add the chromosome numbers of both parent species: Chromosome number of hybrids = Chromosome number of Secale cereale + Chromosome number of Elymus canadensis Chromosome number of hybrids = 14 + 28 = 42 The expected chromosome number in the somatic cells of the hybrids is 42.

(Step 2: Speculate on the anaphase I separation patterns of the chromosomes)

Assuming none of the chromosomes pair at meiosis I in the sterile hybrid, this means that each chromosome will separate independently. During normal meiosis I, homologous chromosomes pair and separate to opposite poles. In this case, however, no pairing occurs for any of the chromosomes. Therefore, there is no organized pattern to the anaphase I separation of the chromosomes in the hybrid. Each chromosome will separate randomly and independently. It is likely that this randomness leads to nonviable gametes or an inability to form healthy offspring, contributing to the sterility observed in these hybrids. In conclusion, the expected chromosome number in the somatic cells of the hybrids is 42, and the anaphase I separation patterns of these chromosomes would be random and independent, with no pairing during meiosis I.

Key concepts

Secale cereale genetics

Understanding the genetics of Secale cereale, commonly known as cereal rye, is crucial when studying plant breeding and hybridization. Secale cereale is a diploid species, which means it has two sets of chromosomes, one from each parent. The chromosome number in Secale cereale is particularly important because it determines how this species can cross with others, especially when it comes to hybrid viability.

When delving into cereal rye's genomics, it becomes evident that the characteristic chromosome number (2n) is 14. This set number provides the basis for the plant's growth, development, and reproduction. It's known that genetic variation within this species can contribute to traits such as yield, disease resistance, and adaptability to different environments. Such knowledge is paramount for breeding programs aiming to develop new rye varieties with desirable characteristics.

Elymus canadensis genetics

On the other side, we have Elymus canadensis, also known as Canadian wild rye, which possesses a chromosome number twice that of Secale cereale – specifically 28 chromosomes. This greater number indicates that Elymus canadensis is a tetraploid species, containing four sets of chromosomes. Tetraploid species can offer greater genetic diversity and may have certain evolutionary advantages, such as increased vigor or adaptability.

Canadian wild rye's genetics have adapted to various North American environments, and its higher chromosome number could be associated with specific growth capabilities and survival strategies. Studying the genetics of Elymus canadensis provides valuable insights into how polyploid plants compare to their diploid counterparts, especially relating to their ecological interactions and adaptability.

Meiosis in hybrids

When diploid and tetraploid species like Secale cereale and Elymus canadensis are crossed, their offspring undergo hybridization, which poses unique challenges during meiosis – the process that leads to the formation of gametes. In typical meiosis, homologous chromosomes (one from each parent) pair up and segregate into gametes to ensure a consistent chromosome number is maintained in the offspring.

However, in hybrids with differing parent chromosome numbers, meiosis can become irregular. Because the chromosomes from each species often do not have exact pairs in the hybrid, they fail to pair correctly during meiosis I, leading to gametes with varying chromosome numbers. This condition often results in sterility due to the production of gametes that do not have the proper number of chromosomes for successful fertilization.

Sterile hybrid chromosome behavior

The hybridization between Secale cereale and Elymus canadensis that results in sterile offspring illustrates the complexities of sterile hybrid chromosome behavior. Without proper chromosome pairing during meiosis, these hybrids are often unable to produce viable gametes. The chromosomes behave independently, leading to a random assortment in gametes which, more often than not, is incompatible with life.

Aside from chromosomal incompatibility, other genetic factors can also contribute to hybrid sterility. These include gene imprinting, translocation events, or inappropriate gene expression levels. Understanding the dynamics of chromosome behavior in sterile hybrids is not only fundamental for genetics but also for developing successful breeding strategies in agriculture and conservation.