Pollination and Hybrid Crops

 

What are Hybrid Crops?:

A hybrid corn crop is produced through controlled cross-pollination of two genetically distinct parent varieties or lines of Zea mays. This breeding process combines desirable traits from both parents, resulting in offspring that exhibit characteristics like improved yield, disease resistance, uniformity, and other beneficial qualities. This high quality of corn traits is due mainly to the genetic diversity that is accomplished through cross-pollination. Hybrid vigor, or heterosis, is a notable advantage where hybrids often show superior traits compared to their parent lines. Hybrid corn is widely used and harvested for its predictability in performance and higher yields, making it a cornerstone of commercial modern agriculture. Unlike open-pollinated varieties, hybrid corn seeds can not reliably be saved and replanted due to the variability of offspring traits. Modern hybrid corn breeding allows for a crop to produce more desirable traits that can be beneficial to the farmer such as drought resistance or tolerance, a better defense against disease, and much more. 

How does pollination work in Hybrid Corn Crops:

Hybrid corn production begins with selecting two genetically distinct parent lines. The two parent lines are chosen based on what they have to offer the final crop, including more enhanced or present desirable traits such as a higher yield or more effective protection. 

Corn plants are monoecious, meaning they have both male and female reproductive structures on the same plant. The tassel produces pollen, where the male gametes are contained, while the ear contains silk strands that lead to the ovules, where one could find the female gametes. 

To ensure controlled cross-pollination, the tassels of one parent line (usually the female parent or seed parent) are often removed before they shed pollen to prevent natural pollination that is more unpredictable. This process is known as detasseling  and is important is controlling the final crop product.

The detasseled plants are then planted alongside the corn plants from the other parent line. This is often considered the male plant as it will produce pollen for fertilization. As the pollen from the second or the male’s tassels matures and sheds, it is carried by wind or gravity to the silks emerging from the ears of the detasseled plants or the female plants.

Each silk on the ear of the detasseled plant receives a pollen grain. The pollen grain germinates on the silk, forming a pollen tube that carries the male gametes located in the pollen down to the ovule where fertilization occurs. This completes the pollination process and develops a kernel.

The resulting kernels on the ear of the detasseled plant are hybrid seeds because of this process with two distinct parent lines. These seeds inherit genetic traits from both parent lines, combining desired characteristics such as high yield, disease resistance, and uniformity due to heterosis.

Hybrid seeds are harvested from the ears of the detasseled plants and are then processed, treated, and sold to farmers for planting in subsequent seasons.

Why can't we use self-pollination in corn crops:

There are some cases where self-pollination could be beneficial for maintaining purity, however, there are many reasons why it might not be preferred. 

The main point of concern is a lack of genetic diversity. The corn plant would be more susceptible to unpredictable or severe weather incidents and would be less equipped to deal with disease attacks. Self-pollination would lead to the offspring plant having the same genes as the parent plant and leaves little to no room for improvement. 

Another point of concern in regards to a lack of genetic diversity is a lower yield being produced by the seed or a lower quality of crop. Self-pollinated corn has been shown to have a lower yield and a less impressive quality than cross pollinated corn. As explained above, this is because genetic variation allows a hybrid seed to have desirable characteristics from two separate parent lines rather than one parent line to which the seed is genetically identical. 

Self-pollination can also have another undesirable outcome known as inbreeding depression. This is when the recessive alleles in a plant can cause the plant to be less fertile, have a lower quality, or be smaller in size over time. This is extremely undesirable commercially because it leads to a reduced kernel weight and quality while leading to a poorer future crop. 

Where cross-pollination can promise a certain degree of uniformity which is ideal for the general corn market, self-pollinated corn can not do the same. This is another reason why it is not preferred. "Uniformity is king in corn" is not an uncommon saying, and displays the importance of having similar kernel weights, sizes, maturity, and growth rates. 

This all does not mean that self-pollination is entirely undesirable. In fact, in order to get hybrid corn it is often important that the two distinct parent lines come from inbred homozygous corn crops, This ensures that the traits are predictable and uniform to better control the traits that the finale hybrid seed might have. Self-pollination can also be beneficial for research purposes. This can help a researcher to better interpret a specific trait or parent line by enhancing its specific characteristics rather than combining it with another parent line. They can also study other aspects of the inbred line's genes such as the gene expression itself and the inheritance patterns. 

When it comes to historical breeds of corn, inbreeding can help to preserve the species itself or specific desirable traits that it may carry. This allows more more production which can benefit Hybrid corn crops as well. In places such as CIMMYT or other seed banks, it can also ensure that there is an adequate seed supply for current and future research purposes. If it was cross pollinated, this would not be the original crop and would greatly affect results in the lab. In addition to these points, inbreeding can help to enhance present traits that can be diminished if cross pollinated with another parent line. 

Self-pollination is desirable for many situations and environments, but in terms of commercial corn production and research development, cross-pollination provides more room for improvement and quality. This is especially true over time, where inbred lines may be depressed, cross-pollinated breeds will display a more diverse genetic makeup.