EXTENSIVE CHROMOSOMAL DIVERGENCE WITHIN A SINGLE RIVER BASIN IN THE GOODEID FISH, ILYODON FURCIDENS

Area of Science:

• Ichthyology
• Cytogenetics
• Evolutionary Biology

Background:

• Viviparous goodeid fish, Ilyodon furcidens, in Mexico exhibit significant variation in metacentric chromosome numbers across populations.
• This variation is categorized into four cytotypes, with one likely representing the ancestral form.

Purpose of the Study:

• To investigate the extent and nature of chromosomal variation in Ilyodon furcidens.
• To determine the relationship between chromosomal divergence, genic variation, and reproductive isolation.

Main Methods:

• Karyotypic analysis to identify and categorize cytotypes based on metacentric chromosome counts.
• Allozyme electrophoresis to assess genic divergence among cytotypes.
• Hybridization experiments (F1, F2, backcross) to evaluate hybrid viability and fertility.

Main Results:

• Four distinct cytotypes were identified, showing a clinal distribution.
• Allozyme comparisons revealed minimal genic divergence (genetic distance = 0.042) among cytotypes, with no fixed allelic differences.
• Intercytotype hybrids were viable and fertile, indicating a lack of significant negative heterosis or reproductive isolation.

Conclusions:

• Chromosomal and genic divergence are uncoupled in Ilyodon furcidens.
• The extensive chromosomal rearrangements observed do not appear to function as significant isolating mechanisms.
• Despite marked chromosomal evolution, speciation has not been fostered in this lineage.