PHYLOGENETIC IMPLICATIONS OF DIVERSE SEED TYPES, CHROMOSOME NUMBERS, AND POLLEN MORPHOLOGY IN HOUSTONIA (RUBIACEAE)

Area of Science:

• Botany
• Plant Morphology
• Cytogenetics

Background:

• The genus Houstonia (including Hedyotis) comprises numerous North and Central American species.
• Previous taxonomic understanding has been limited by incomplete morphological and cytogenetic data.

Purpose of the Study:

• To conduct a comprehensive taxonomic revision of 39 Houstonia species using seed and pollen morphology.
• To generate new chromosome count data for understudied taxa.
• To establish phylogenetic relationships within the genus based on integrated data.

Main Methods:

• Light microscopy and scanning electron microscopy were employed to analyze seed and pollen external morphology.
• Chromosome counts were performed for eight selected taxa.
• Data on seed, pollen, and chromosomes were correlated to define species groups.

Main Results:

• Significant diversity in seed external morphology (compression, margins, testa, ventral cavities, hilar ridges) was observed.
• Three distinct pollen aperture types were identified, with a novel advanced type found in H. caerulea and related species.
• New chromosome counts were obtained for five taxa, including a new base number (n=17) for Houstonia gracilis. The genus now shows known chromosome numbers ranging from x=6 to x=17.

Conclusions:

• The integrated analysis of seed, pollen, and chromosome data supports the arrangement of 39 species into twelve distinct groups.
• Geographic distribution further corroborates the integrity of six major species groups.
• Further cytogenetic data are needed for five monotypic groups to resolve taxonomic uncertainties and inform future phylogenetic analyses.