Evolution of Male Tails

The evolution of male tail morphology has been our primary focus:

MH27 monoclonal antibody, a tool for observing cell boundaries in Rhabditidae

Evolution of male tail tip morphogenesis and analysis of "Leptoderan" and "Peloderan" character states

The evolution of novel male copulatory behaviors correlates with the evolution of novel male tail morphologies

MH27: a tool for seeing apical cell boundaries

MH27 antibody recognizes an epitope in the belt adherens junctions that surround the apical surfaces of the hypodermal and sensilla cells.
Using MH27 immunofluorescenc staining, we can trace the boundaries of cells as they appear, die, fuse together, or change position during male tail development.
For a model of the 4 tail tip cells showing the location of the adherens junctions, click here.

Homologies for male tail sensilla

The sensilla. In general, males in Rhabditidae have 9 bilateral pairs of genital papillae ("rays") required for finding the vulva during copulation. Both females and males have a pair of "phasmids", which appear to be chemoreceptive sensilla.
Classical assumptions about homologies of the ray and phasmid sensilla in the male tail were based on morphological observations of adults only.
The developmental origins of these sensilla can now be observed at the single cell level using MH27.
In all Rhabditidae we have analyzed so far, the cells that form the rays (as well as other parts of the male tail hypodermis) originate in the same relative positions. We call this conserved pattern the rhabditid male tail Archetype (see figure). (See also Fitch 1997.)
During the evolution of male tails, the positions of homologous rays in adult males of different Rhabditidae species have changed substantially (see figure), even though the relative positions of ray cell origins has not changed at all. That is,

Many ray cells migrate after they are born.
Evolutionary changes have resulted in different patterns of migration and cell-cell association (see Fitch & Emmons 1995).

Reconstructing the pattern of evolutionary change in sensilla patterning reveals some changes that are strikingly similar to the effects of mutations in C. elegans patterning genes, suggesting possible candidate genes that could have been involved in these evolutionary changes (see Fitch 1997).

Evolution of male tail tip morphologies

Tail tips in Rhabditidae have been classically categorized into 2 states: "Leptoderan" (pointy) or "Peloderan" (blunt-ended).
The cellular morphology of tail tips are now being studied with MH27 and electron microscopy (EM).
At least one evolutionary change in male tail tip morphology occurred from peloderan to leptoderan in the "Eurhabditis" group of Rhabditidae (see figure).
This evolutionary change is strikingly similar to the effects of mutations we have generated in C. elegans that result in leptoderan tail tips (e.g., lep-1 mutants), suggesting possible candidate genes that could have been involved in these evolutionary changes (see Fitch 1997).
Contrary to the classic, dichotomous nomenclature for tail tip states, we find that not all peloderan tails are constructed in the same way; neither are all leptoderan tails (see figure). Our MH27 and EM studies, in the context of phylogenetic analysis, are helping us to define the states of tail tip characters.

Male tail form and copulatory behavior: evolutionary correlates

Our phylogenetic analysis has revealed an evolutionary correlation between one aspect of male tail form and function: big bursae are required for parallel mating (see figure).
Ancestrally, males of Rhabditidae species did not have broad bursae ("fans"), and mating position was spiral.
A bursa evolved once, but parallel mating did not arise immediately (some species with bursae still show spiral mating).
Parallel mating arose later only in lineages where males already had bursae.
In lineages where bursae were secondarily reduced, parallel mating was lost, and spiral mating was again used.
Sometimes, parallel mating is lost even in lineages that retain bursae.
Taken together, these data suggest that a bursa (fan) is preadaptive to parallel mating, and that a capacity for spiral mating is often retained.
We are currently seeking correlations between microhabitat and mating position to see if substrate type may result in selection for a particular mating position.


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