The Yrythny were a race native to the Gamma Quadrant, their homeworld was called Vanimel. They were primarily aquatic in nature, spawning in water and moving to land as they grew older. The Yrythny were divided into two castes, "Houseborn" and "Wanderer".
As of 2376 the Yrythny system was under blockade by the Cheka who deployed nano-web weapons in random hidden positions around the Yrythny system in an attempt to force complicity. The Cheka were attempting to gain access to unfertilized Yrythny eggs for genetic research and experimentation. (DS9 novel: This Gray Spirit)
Sometime in the distant past, the Yrythny genetic structure was artificially modified by an unknown race, to accelerate their evolution. This resulted in their genetic structure containing a unique "Turn Key" component in their DNA which could potentially be applied to a multitude of other races.
In the last decade, the Cheka discovered the information about this Turn Key and attempted to force the Yrythny to hand over their young and citizens for experimentation. The Cheka were attempting to produce a genetically engineered servitor race and had expected to use the Yrythny DNA as a template. However, due to the Cheka willingness to experiment on the living with tortuous and malicious methods, the Yrythny refused to cooperate resulting in a Cheka blockade of the planet.
In 2376, the USS Defiant made first contact with the Yrythny after falling victim to a Cheka weapon, the Defiant was towed to the Yrythny homeworld for repairs. During this time, the Yrythny council enlisted Ezri Dax as a mediator between the two castes of Yrythny society.
It was later discovered that Wanderers, and their descendants were actually the next step in Yrythny evolution and the Houseborn Yrythny were in fact on a path to die out if they did not breed with Wanderers. If the current sociological progression continued without change to introduce equality for Wanderers, Dax and Thirishar ch'Thane theorized that within a few hundred years, the Yrythny would suffer a similar reproductive crisis to that which the Andorians faced.
By the time Defiant had left the system to resume its mission, the Yrythny had a viable defence against the Cheka blockade weapons, and were seeking to break down caste restrictions in colony worlds they intended to seed in other star systems. (DS9 novel: This Gray Spirit)
By 2381 the Turn Key DNA of Yrythny eggs had been used in an attempt to improve Andorian fertility, however most of the offspring suffered from genetic problems and stillbirth. (ST - Typhon Pact novel: Paths of Disharmony)
The Yrythny were divided into two castes, which were based on how an individual Yrythny was spawned. The Houseborn Yrythny were those who were spawned by a member of a house and were able to return to their spawn location once they had come of age to return to land. The Wanderers however, were Yrythny young who could not return to their spawning location and were abandoned. Wanderer young who washed up on the shores of a strange house would be taken in as servants and slaves, however in the past Houseborn would club a Wanderer youngling to death.
Wanderer's were considered to be genetically inferior and defective and were not allowed to reproduce, however many Wanderers hosted illicit breeding grounds hidden from the Houseborn to continue their lines. These acts were considered illegal by the Houseborn who considered most Wanderers to be terrorists and agitators. However, Wanderer were naturally more creative, expressive and artistic then their Houseborn compatriots.
Houseborn Yrythny controlled most of the planet and held onto key areas of government, military and business. Houseborn and Wanderers were forbidden to mate in any way. Anyone found doing so would be put to death or sentenced to life in prison, with their young killed. (DS9 novel: This Gray Spirit)