Prof research peeks into early evolution

September 17, 2012  |  
Xiangjia Min is a biology professor at Youngstown State University who is doing research on the sex chromosomes of the papaya plant.

Xiangjia “Jack” Min with a papaya, the focus of his research.

Xiangjia “Jack” Min, assistant professor of Biology, recently participated in a research study to help papaya growers produce a variety that will bear better, more consistent fruit.

The research findings are included in two papers in the Proceedings of the National Academy of Sciences and was featured this summer in an article titled “Researchers Peek at the Early Evolution of Sex Chromosomes,” published in Science Daily.

Min, who joined the YSU faculty in 2008, says the research will continue and is sure to produce more studies on chromosomal research.

“The far reaching implications are interesting, not just for plant biology, but also for human and other species’ evolution,” said Min.

The Papaya plant reproduces sexually, meaning there are male (XY) and female (XX) papaya trees, but there is also a hermaphroditic-type (XYh) with the attributes of both genders. Unlike male or female plants, it self-pollinates, producing fruit by itself (an individual tree). This is the papaya farmer’s preferred type, but due to segregation, about one-third of female trees are produced from the seeds of hermaphrodites. This means papaya growers have no way to consistently produce the desired hermaphroditic papaya plant.

“The evolution of the chromosomes is what has created the diversity among species,” said Min. “The significance of the research is that it could be a general mechanism for sex chromosomal evolution in other species.”

The research project was led by Ray Ming, professor of plant biology at the University of Illinois at Urbana-Champaign. He and a group of researchers from multiple universities tackled the project by sequencing the hermaphrodite-specific region of the Yh chromosome and its X chromosome counterpart and have now moved closer to solving the problem.

For the data sequencing analysis, Ming invited Min, a fellow University of Hawaii graduate, to participate in decoding the molecular mechanism of XY chromosome evolution.

Min, a specialist in bioinformatics — a combination of biology and computer science — used computer software to analyze gene and genome sequences looking for events that may be the cause of the evolution of XY chromosomes in this species.

Data comparisons found that the female sex chromosome region had a shorter DNA sequence of about 3.5 million DNA base pairs, compared with 8.1 million base pairs for the hermaphrodite-specific regions of the Yh chromosome.

An older inversion, which occurred about 7 million years ago in the identified two large-scale inversions in the Yh chromosome, might be the likely mechanism that caused the suppression of recombination between X and Yh, leading to X and Yh evolution.

The extra DNA sequences in the Yh were caused by accumulation of retrotransposons — abundant genetic elements that can amplify themselves in genomes — and repetitive DNA sequences. Both Y-specific and X-specific genes are identified. This is how the distinguishing factors between the hermaphrodite variety and the female plant were identified.

The research will help move papaya growers one step closer to creating a hermaphrodite without a Y chromosome and, ultimately, a true-breeding hermaphrodite.

“There are other implications for the theoretical study of sex chromosomal evolution in other species,” said Min. “While there is no way to travel 167 million years to study the human X chromosome, this research will help us to make some very educated guesses.”

Story by Robert Merz